All three vaccines used in the United States — made by Pfizer-BioNtech, Moderna and Johnson & Johnson — remain highly effective at preventing serious illness, hospitalization and death from Covid-19. But some recent studies have shown differences in efficacy. The findings don’t translate to a meaningful difference in protection in the real world, because all of the vaccines work well. But the research could influence decisions about the timing of booster shots.

One study evaluated the real-world effectiveness of the Pfizer and Moderna vaccines at preventing symptomatic illness in about 5,000 health care workers in 25 states. The study found that the Moderna vaccine had an effectiveness of 96.3 percent, compared with Pfizer’s 88.8 percent. A study conducted by the Centers for Disease Control and Prevention found that Moderna’s protection against hospitalization didn’t wane after four months, but Pfizer’s fell to 77 percent from 91 percent.

In a different study, the effectiveness of the Johnson & Johnson vaccine against hospitalization remained steady five months after vaccination, at around 81 percent. —Apoorva Mandavilli

Read More Moderna vs. Pfizer: Both Knockouts, but One Seems to Have the Edge

Yes. You should never have to pay anything out of pocket to get a Covid vaccine, although you will be asked for insurance information. If you don’t have insurance, you can still get the vaccine at no charge. If you get your vaccine from a doctor’s office or an urgent care clinic, ask about potential hidden charges. To be sure you won’t get a surprise bill, the best bet is to get your vaccine at a health department vaccination site or a local pharmacy. —Sarah Kliff

Read More The Vaccines Are Supposed to Be Free. Surprise Bills Could Happen Anyway.

Shots are given at local pharmacies, state health department vaccination centers and doctors’ offices. Check the websites of your state’s health department or local pharmacy chains for more details. You can schedule an appointment online or, in some cases, just drop in, although the recent approval of booster shots for eligible people could increase the wait time.

A Covid vaccine is free, but bring your insurance card if you have one. You may be asked for identification, but in most cases you won’t be turned away if you don’t have it. Call ahead to be sure. Plan to spend about 30 minutes at an appointment, including a 15- to 30-minute waiting period after you get a shot for observation. You’ll be given a vaccine card at the end of the appointment that will include the date to return for your second dose, if required. —Tara Parker-Pope

The jab of the needle feels like any other vaccination. Sore arms after the shot are common. Many who have received the vaccine likened the arm pain following the injection to that of a flu shot; for others, it was considerably worse. The New York Times interviewed several dozen of the newly vaccinated about how they felt in the days afterward. They recounted a wide spectrum of responses, from no reaction at all to symptoms like uncontrolled shivering and “brain fog.” —Amy Harmon

Read More What the Vaccine Side Effects Feel Like, According to Those Who've Gotten It

The online scheduling systems should tell you which vaccine you’re signing up for, or you can call ahead to make sure the vaccination site is dispensing the one you want. While the major pharmacies are offering all three vaccines, most locations will have only one type available at a specific site. —Tara Parker-Pope

Everyone who gets the vaccine will be asked to stick around for about 15 minutes after getting the shot. This will allow health workers to monitor you for any signs of an allergic reaction, which are rare. A person with a history of severe allergies may be asked to stay for 30 minutes. Make sure you bring a quality mask to your vaccination appointment to wear while you wait. Even though you just received the vaccine it will take a few weeks after your final dose before you are protected. —Tara Parker-Pope

Both the vaccines from Pfizer-BioNTech and from Moderna have two doses. Pfizer-BioNTech’s second dose comes three weeks after the first, and Moderna’s comes four weeks later. The second dose provides a potent boost that gives people strong, long-lasting immunity. You should try to stick as closely as possible to the prescribed schedule. If scheduling conflicts prevent you from coming back for the booster shot on the exact day it’s due, federal health officials say it's okay to get the second dose up to four days early or up to six weeks after the first dose. If you miss this window, you should still come back for the second dose as soon as you can. —Carl Zimmer, Tara Parker-Pope

The U.S. Centers for Disease Control has said a person is fully vaccinated two weeks after receiving the final dose of whatever vaccine you’ve been given. If your vaccine requires only one dose, it will take about two weeks for your body to build a strong immune response. If your vaccine requires two doses, the peak vaccine response won’t be reached until two weeks after the second dose. —Carl Zimmer and Denise Grady

Read More Pfizer’s Vaccine Offers Strong Protection After First Dose

The Food and Drug Administration authorized booster shots for a select group of people who received their second doses of the Pfizer-BioNTech vaccine at least six months ago. Eligible people include Pfizer recipients who are 65 or older or who live in long-term care facilities. The agency also authorized boosters for adults who are at high risk of severe Covid-19 because of an underlying medical condition, as well as for health care workers and others whose jobs put them at risk.

People with weakened immune systems who received the Pfizer or Moderna vaccines are also eligible for a third shot at least four weeks after their second doses. For these patients, the third shot is not considered a booster dose — it is now part of the recommended immunization schedule for those with compromised immune systems who don’t generate a robust response after just two shots. —Tara Parker-Pope

Read More Answers to Your Questions About Booster Shots

Regulators are expected to authorize booster shots for recipients of the Moderna and Johnson & Johnson vaccines soon. An F.D.A. advisory panel has voted in favor of emergency authorization of a half-dose booster of Moderna vaccine, at least six months after the second dose. Those eligible would include Moderna recipients over 65 and other adults considered at high risk — the same groups eligible for a Pfizer-BioNTech booster. The same expert committee voted unanimously in favor of authorizing booster shots for all adults (roughly 15 million people) who received the single-dose Johnson & Johnson vaccine. The second J&J dose would be given at least two months after the first dose. —Carl Zimmer

Read More An extra J. & J. shot substantially boosts protection against Covid, the company reports.

The C.D.C. has said the conditions that qualify a person for a booster shot include:

• Hypertension and heart disease
• Diabetes or obesity
• Cancer or blood disorders
• Weakened immune system
• Chronic lung, kidney or liver disease
• Dementia and certain disabilities

Pregnant women and current and former smokers are also eligible. —Tara Parker-Pope

Read More Answers to Your Questions About Booster Shots

No. The C.D.C. has recommended booster shots only for people 65 or older, those in long-term care facilities, and people 50 or older who are at high risk.

The agency has not given advice to others who are eligible, saying only that at-risk adults 18 to 49 or those in frontline jobs should evaluate their individual risk, or discuss the issue with a health care provider, before getting a booster shot. —Tara Parker-Pope

Read More Answers to Your Questions About Booster Shots

The F.D.A. authorized booster doses for workers whose jobs put them at high risk of exposure to potentially infectious people. The C.D.C. says that group includes:

• Emergency medical workers (health care workers, firefighters, police officers, employees at congregate care facilities including those who work in homeless shelters)
• Education workers (teachers, support staff workers, day care workers)
• Food and agriculture workers
• Manufacturing workers
• Corrections workers
• U.S. Postal Service workers
• Public transit workers
• Grocery store workers

The C.D.C. has said the list may change over time. —Tara Parker-Pope

Read More Answers to Your Questions About Booster Shots

Health departments, pharmacies and doctors’ offices will dispense boosters much the same way that they administered the first and second doses. Call ahead to find out about scheduling, and bring your vaccine card. Proof of an underlying medical condition won’t be required.

You will be able to find more information about getting a booster shot on your state’s health department website or pharmacy websites. You can also talk to a health care provider about whether you should get a third shot. Since the F.D.A. fully approved the Pfizer-BioNTech vaccine as a two-dose regimen this past summer, physicians have had broad latitude to prescribe a third dose to people they deem in need of one. —Tara Parker-Pope

Read More Answers to Your Questions About Booster Shots

While people who are severely immune-compromised can get a third shot sooner, everyone else who qualifies should wait until at least six months after getting a second shot. Getting a booster too soon is probably a waste of a dose and may not increase your antibodies in a meaningful way; there is also a lack of data on the safety of booster shots given early. —Tara Parker-Pope

Read More Answers to Your Questions About Booster Shots

While data is limited, so far reactions reported after the third mRNA dose from Pfizer or Moderna are similar to those experienced after the two-dose series: Fatigue and pain at the injection site are the most commonly reported side effects, and, overall, most symptoms are mild to moderate, the C.D.C. says. A survey from Israel, where booster shots are already being given, found that 88 percent of Pfizer vaccine recipients said that in the days after the third dose they felt “similar or better” to how they felt after the second shot. About a third of respondents reported some side effects, with the most common being soreness at the injection site, and 1 percent said they sought medical treatment because of one or more side effects. —Tara Parker-Pope

Read More Answers to Your Questions About Booster Shots

It is not recommended yet, although new guidance is expected soon. The F.D.A. is reviewing preliminary data suggesting that Johnson & Johnson recipients may be better off with a booster shot from Moderna or Pfizer. —Tara Parker-Pope

Read More Answers to Your Questions About Booster Shots

While the Biden administration has said it supports booster shots for everyone who is eight months post-vaccination, the plan has been rejected by F.D.A. scientists. But the recommendation could change in the coming weeks or months as more data becomes available on the durability of vaccine antibodies over time. The good news is that the consensus in the scientific community is that all of the vaccines continue to provide strong protection against severe illness, hospitalization and death from Covid-19. —Tara Parker-Pope

Read More Answers to Your Questions About Booster Shots

Yes. The C.D.C. says the Covid vaccine may be administered without regard to the timing of other vaccines, and many pharmacy sites are allowing people to schedule a flu shot at the same time as a booster dose. Side effects are generally similar when the vaccines are given simultaneously as when they are administered separately. If you do get a flu shot and a Covid vaccine at the same time, experts advise using different arms to avoid soreness, or at least spacing the injection site for each shot by at least one inch. —Christine Hauser

Read More It's Time to Get a Flu Shot

The Food and Drug Administration so far has given emergency authorization only to the Pfizer-BioNTech vaccine for young people aged 12 to 17. Moderna and Johnson & Johnson are expected to seek emergency authorization for children in the coming months. —Tara Parker-Pope

The date for when a vaccine will be approved for children ages 5 to 11 has been something of a moving target. Pfizer has asked the F.D.A. for authorization to use its vaccine in this younger age group. If the regulatory review follows a timeline similar to the ones for older children and adults, it’s possible that millions of elementary school students could be inoculated before Halloween. But public health officials have said it may take longer to review the data, with estimates for authorization ranging from late fall to the end of 2021.

Vaccines for very young children, ages 6 months to 4 years old, aren’t expected until 2022. Trial results for children younger than 5 are not expected until the fourth quarter of this year at the earliest, according to Dr. Bill Gruber, a senior vice president at Pfizer and a pediatrician. —Tara Parker-Pope and Apoorva Mandavilli

The dosage of Pfizer vaccine authorized for 12- to 17-year-olds is identical to the dosage given to adults: two doses of 30 micrograms given three weeks apart.

The dose for children ages 5 to 11 is expected to be just 10 micrograms per shot — a third of the dose given to older children and adults. The children who got the lower dose of vaccine produced a response comparable to the levels of antibodies seen in the earlier trials of participants ages 16 to 25. In children younger than 5, just three micrograms — one-tenth of the adult dose — is being tested in trials.

Moderna is studying different dosing strategies for its vaccine in 6,750 healthy children in the United States and Canada. In adults, the standard dose is 100 micrograms given four weeks apart. In Moderna’s study of children ages 2 to 11, the company is testing doses of either 50 or 100 micrograms. In children under 2, Moderna is studying shots of 25, 50 or 100 micrograms. —Apoorva Mandavilli

Children who are eligible will be able to receive a shot from their pediatricians, local pharmacies or school-based clinics. Check your local health department’s website or ask your pediatrician for details. Some sites may require appointments, while others will offer shots on a walk-in basis. —Tara Parker-Pope

Yes. The C.D.C. says Covid vaccines and other vaccines can be given without regard to timing. The agency says that if multiple vaccines are administered during a single visit, the injections may be given in different parts of the body.

“Experience with other vaccines has shown that the way our bodies develop protection, known as an immune response, after getting vaccinated and possible side effects of vaccines are generally the same when given alone or with other vaccines,” the C.D.C. says. —Tara Parker-Pope

Detailed side effect data for children ages 5 to 11 hasn’t been released. In studies of 12- to 15-year-olds, fevers were slightly more common in children compared with adults. But in general, the side effects reported in children have been similar to those seen in older people. The F.D.A. said that the most commonly reported side effects in the adolescent clinical trial participants were pain at the injection site, tiredness, headache, chills, muscle pain, fever and joint pain. Side effects typically lasted one to three days. Although pain at the injection site was common after both shots, more adolescents reported side effects after their second doses. Younger people tend to have a more powerful immune response than older people because they have more robust immune systems. It’s possible that children may experience more side effects than their parents did from the same shot.

Children should not get the Pfizer vaccine if they have a history of severe allergic reaction to any ingredient (such as polyethylene glycol) in that vaccine. Allergies to the vaccine ingredients are rare. The vaccine does not contain eggs, preservatives or latex. If you have doubts or aren’t sure, talk to your pediatrician before having your child vaccinated. If your child has severe allergies to anything else (medications, foods, bees), remain at the vaccination site for 30 minutes after the injection, instead of the 15 minutes that the general population is recommended to wait. —Tara Parker-Pope

The answer depends on whether your child attends public or private school. Private schools, day care centers and camps can decide whether to require students to be vaccinated against Covid-19 as a condition of returning to school or the facility. So far more than 800 colleges and universities have adopted campus-wide vaccine mandates.

For public K-12 schools, vaccination requirements are largely left up to the states. All 50 states have legislation requiring specified vaccines for students, although no state currently requires children to receive the Covid-19 vaccine as a condition of returning to school, according to the National Conference of State Legislatures. Exemptions to school immunization requirements vary, but all states grant exemptions to children for medical reasons. Children of parents who have religious objections to immunizations can receive a religious exemption in 44 states and Washington, D.C. And 15 states allow philosophical exemptions for children whose parents object to immunizations because of personal, moral or other beliefs.

Los Angeles is the first major school district in the United States to mandate coronavirus vaccines for students 12 and older who are attending class in person, but legal challenges are expected. —Tara Parker-Pope

Read More Los Angeles Mandates Vaccines for Students 12 and Older

Pfizer’s trial included 2,268 children ages 5 to 11, two-thirds of whom received two doses of the vaccine three weeks apart; the rest were injected with two doses of saltwater placebo. Given how rarely children become severely ill, the trial was not big enough to draw meaningful conclusions about the vaccine’s ability to prevent Covid or hospitalization. Instead, the researchers relied on measurements of the children's immune response, on the assumption that the protective levels of antibodies seen in older people would be as protective in younger children.

The Pfizer study for older children enrolled 2,260 participants ages 12 to 15. Of those children, 1,131 received the vaccine (two shots, given three weeks apart) and 1,129 received saline placebo shots. The vaccine worked even better in children than it did in adults. No children in the vaccine group got sick with Covid-19, while 18 children in the placebo group became ill. The company is still gathering information, including testing the trial participants every two weeks for the coronavirus.

Moderna recently released the results of its trial testing the vaccine in 3,732 people ages 12 to 17, two-thirds of whom received two vaccine doses. There were no cases of symptomatic Covid-19 in fully vaccinated adolescents, the company reported.

The F.D.A. has asked the companies to include 3,000 children in tests of 5- to 11-year-olds. Beyond the clinical trials, health officials are continuing to gather information on the 12.7 million young people ages 12 to 17 in the United States who have received at least one dose of the Pfizer shot, according to C.D.C. data. —Tara Parker-Pope

Given that the vaccine’s mRNA molecule mimics a natural human process, experts say they are confident that the vaccines are safe for growing bodies. Dr. Paul Offit, director of the vaccine education center at Children’s Hospital of Philadelphia and a member of the Food and Drug Administration’s vaccine advisory panel, notes that while mRNA vaccines are new, mRNA molecules occur naturally throughout the human body.

“Every child in their cells has about 200,000 copies of messenger RNA,” said Dr. Offit. “Every cell in your body has these molecules which are making proteins and enzymes so you can continue to live. Although the technology is new for a vaccine, it’s not like it’s a molecule we haven’t seen before. I understand the anxiety, but it’s no different than when you make insulin or hemoglobin or albumin or any of the other proteins your body makes.”

One reassuring fact about the mRNA vaccines is that the molecule is destroyed by the cell once it completes its mission, so it doesn’t stay in the body. Another common worry among parents is the effect of a new drug or vaccine on brain development. Dr. Offit noted that the body has a blood-brain barrier that prevents most proteins from entering the brain. “Your brain is an immunologically protected site,” he said.

Although mRNA technology has been studied for about 15 years, this is the first time it has been used in a vaccine. It’s also being studied to treat cancer, muscular dystrophy and other diseases. —Tara Parker-Pope

While children are less likely to develop severe illness from Covid-19, they are still at risk. In early September, children accounted for nearly 30 percent of coronavirus cases, and the highly contagious Delta variant has sent more children into hospitals and intensive care units in the past few weeks than at any other time during the pandemic. This past winter, doctors reported growing numbers of patients with Multisystem Inflammatory Syndrome in Children, or MIS-C, a condition linked to Covid which can affect multiple organs, including the heart.

Unvaccinated children, even if they do not become ill, can spread the virus to family members, teachers and others they interact with regularly, including grandparents or others who are at higher risk for severe disease or death. —Tara Parker-Pope

Read More Hospitalizations for Children Sharply Increase as Delta Surges

The stab of the needle into your arm won’t feel different than any other vaccine that requires an injection. But the rate of short-lived side effects from Covid-19 vaccines does appear higher than that of a flu shot. Sore arms are common. Millions of people have already received the vaccines, and the overwhelming majority have not reported any serious health problems. (Severe allergic reactions have occurred in a fraction of cases. You can read more in the next question.) Side effects, which sometimes can resemble the symptoms of Covid-19, last about a day and appear more likely after the second dose. Early reports suggest some people feel lousy and might need to take a day off from work after receiving the second dose. In the Pfizer study, about half developed fatigue. Other side effects occurred in at least 25 to 33 percent of patients, sometimes more, including headaches, chills and muscle pain.

While these experiences aren’t pleasant, they are a good sign that your own immune system is mounting a potent response to the vaccine that will provide long-lasting immunity. The New York Times interviewed several dozen of the newly vaccinated in the days afterward. They recounted a wide spectrum of responses, from no reaction at all to symptoms like uncontrolled shivering and “brain fog.” As vaccines go, experts have agreed, the two Covid vaccines being distributed now elicit more reactions than most. "We call them ‘side' effects, but it’s really just an effect,” said Dr. Paul Offit, who is a member of the Food and Drug Administration’s vaccine advisory panel. “This is what your immune response does when it’s responding to an infection.” —Abby Goodnough and Amy Harmon

Read More What the Vaccine Side Effects Feel Like, According to Those Who've Gotten It

While severe allergic reactions, called anaphylaxis, have occurred in the minutes following an injection with the new vaccines, it’s an extremely rare event. The rate of anaphylaxis has been 11.1 cases per million doses, as of December. By comparison, the rate of severe allergic reaction to the flu shot is about 1.35 cases per million doses. While those statistics may sound scary, severe allergic reactions are a risk with many drugs. Dr. Aaron Carroll, a professor of pediatrics at Indiana University School of Medicine, notes that the risk of anaphylaxis from penicillin drugs is even higher — between one in 2,500 and one in 5,000 — but that doesn’t stop doctors from prescribing them. You can read more from Dr. Carroll about the risks of vaccines here.

All the patients who experienced severe reactions to the vaccines were treated and have recovered. Most of them had a history of anaphylaxis or allergies, but some patients had no known allergies. Every vaccination site is required to have epinephrine and other emergency supplies in case a patient has an allergic reaction. (If you normally carry an EpiPen, bring it to your vaccination.) If you have a history of anaphylaxis, you’ll be monitored for 30 minutes after the shot, instead of the standard 15 minutes.

If you’ve ever had anaphylaxis for any reason, or an allergic reaction to a vaccination, you should talk to your doctor about how to safely get the vaccine and what precautions to take. You may be advised to schedule your shot in a hospital or close to a health care facility. If you had an allergic reaction to your first dose of Covid vaccine, you’ll be advised to skip the second dose. People who have had allergic reactions to either of two ingredients — polyethylene glycol or polysorbate — are also being warned not to receive a vaccine. —Tara Parker-Pope

Read More Here’s What People With Allergies Should Know About Covid Vaccines

Side effects like fatigue, headaches and muscle pain should show up within one to three days after vaccination, and resolve one to three days after they start. Sore arms and fevers are common. Contact your doctor if the redness or tenderness where you got the shot increases after 24 hours. You should also call your doctor if your side effects are causing you worry, or they do not seem to be going away after a few days.

Dr. Sylvia Owusu-Ansah, an emergency physician in Pittsburgh, chronicled her vaccine side effects on Facebook. She experienced mild muscles aches and a sore left arm on Day 1 but was still experiencing nasal congestion and mild headache on Days 3 through 5. Along with a card reminding you to get the necessary second dose, vaccine recipients are handed information on how to report side effects to the Centers for Disease Control and Prevention through an app called V-Safe, a smartphone-based tool that uses text messaging and web surveys to provide personalized health check-ins after you receive a Covid-19 vaccination. —Dani Blum and Amy Harmon

Short-lived side effects like fatigue, headache, muscle aches and fever are more common after the second dose of both the Pfizer-BioNTech and the Moderna vaccines, which each require two shots. (The Johnson & Johnson vaccine requires only a single shot.) Patients who experience unpleasant side effects after the second dose often describe feeling as if they have a bad flu and use phrases like “it flattened me” or “I was useless for two days.” During vaccine studies, patients were advised to schedule a few days off work after the second dose just in case they needed to spend a day or two in bed.

Data collected from v-safe, the smartphone-based tool everyone is encouraged to use to track side effects after vaccination, also show an increase in reported side effects after the second dose. For instance, about 29 percent of people reported fatigue after the first Pfizer-BioNTech shot, but that jumped to 50 percent after the second dose. Muscle pain rose from 17 percent after the first shot to 42 percent after the second. While only about 7 percent of people got chills and fever after the first dose, that increased to about 26 percent after the second dose. —Tara Parker-Pope

Read More What the Vaccine Side Effects Feel Like, According to Those Who’ve Gotten It

Most experts agree it’s safe to take a pain reliever or fever reducer like acetaminophen or ibuprofen to relieve discomfort after you get vaccinated. You shouldn’t try to stave off discomfort by taking a pain reliever before getting the shot.

The concern about whether pain relievers might dampen the effect of the vaccine stems from research in pediatric patients. Parents sometimes give children pain relievers like acetaminophen or ibuprofen before and after they get vaccinated to reduce fevers and aches that might occur following childhood vaccinations. Because fevers and other side effects are also a sign that the body is mounting a strong immune response, some researchers have questioned whether giving a child a pain reliever or fever reducer before or after a shot might blunt the effectiveness of the vaccine.

A review of studies of more than 5,000 children compared antibody levels in children who took pain relievers before and after vaccinations and those who did not. They found that pain relievers did not have a meaningful impact on immune response, and that children in both groups generated adequate levels of antibodies after their shots. Another study looked specifically at giving 142 children acetaminophen, ibuprofen or a placebo after a flu shot. The vaccine response was not significantly different in patients taking pain relievers or the placebo.

Neither Pfizer nor Moderna offers guidance about taking pain relievers to treat side effects. A tip sheet from the Centers for Disease Control and Prevention suggests talking with a doctor before taking an over-the-counter pain reliever after your vaccine.

An after-care guide from the British Columbia Centre for Disease Control is more specific about treating discomfort with over-the-counter pain relievers. “You may feel unwell for a day or two,” the guide states. “If you are unable to carry on with your regular activities because of these symptoms, you can take medication such as acetaminophen or ibuprofen. Check with your health care provider if you need advice about medication.”

Several medical and health groups, including the Henry Ford Health System and UCI Health , advise against taking prophylactic pain relievers before your shot, but they agree it’s fine to take an over-the-counter pain reliever for discomfort after getting the vaccine.

“Taking over-the-counter medications such as acetaminophen and ibuprofen before receiving a vaccine may reduce its ability to work and blunt your immune response to the vaccine,” advises UCI Health. “After the vaccination, don’t hesitate to take an over-the-counter medication if you have symptoms that make you uncomfortable." —Tara Parker-Pope

Just as some people experience side effects from medications and some don’t, people have varied reactions to vaccines. While we tend to hear only about the unpleasant reactions after the vaccine, a lot of people experience only mild discomfort or no symptoms at all after getting the shot.

In the Pfizer trial, for instance, about half the participants developed fatigue. Other side effects occurred in at least 25 to 33 percent of patients, including headaches, chills and muscle pain. That means that half or more of the participants did not have those side effects, and yet the overall efficacy of the vaccine was 95 percent, suggesting that a lack of side effects does not mean a vaccine isn’t working. We also know that older people tended to report fewer side effects than younger people, probably because aging immune systems aren’t as strong. As people age, bodily defenses against pathogens weaken, and the response to vaccines also falters. But in the Pfizer and Covid vaccine trials, older people still produced adequate levels of antibodies, indicating a strong immune response after the vaccine. If you don’t have side effects after your shot, be glad you are one of the lucky ones and don’t worry. —Tara Parker-Pope

An analysis from the first 13.7 million Covid-19 vaccine doses given to Americans found that side effects were more common in women. And while severe reactions to the Covid vaccine are rare, nearly all the cases of anaphylaxis, or life-threatening allergic reactions, occurred in women.

The finding that women are more likely to report and experience unpleasant side effects to the Covid vaccine is consistent with other vaccines as well. Women and girls can produce up to twice as many antibodies after receiving flu shots and vaccines for measles, mumps and rubella (M.M.R.) and hepatitis A and B. One study found that over nearly three decades, women accounted for 80 percent of all adult anaphylactic reactions to vaccines.

While it’s true that women may be more likely to report side effects than men, the higher rate of side effects in women also has a biological explanation. Estrogen can stimulate an immune response, whereas testosterone can blunt it. In addition, many immune-related genes are on the X chromosome, of which women have two copies and men have only one. These differences may help explain why far more women than men are afflicted with autoimmune disease, which occurs when a robust immune response attacks the body’s healthy tissue. —Tara Parker-Pope

Read More The Most Common Questions About Vaccination Side Effects, Answered

A rare side effect of the vaccine has been seen in a few people who have previously been injected with dermal fillers, also called “wrinkle fillers,” which are gel-like substances used to smooth wrinkles and facial lines around the nose and mouth, plump lips and restore volume to sunken cheeks.

In a few cases, people have developed swelling in the parts of the face that had been treated with the fillers. One to two days after getting the vaccine during the Moderna clinical trials, three women (out of 15,184 people who received at least one dose of the vaccine) developed swelling where they had previously been injected with cosmetic fillers. A 29-year-old woman developed swelling in her lips two days after the vaccine, and reported she had previously had a similar reaction to the flu shot.

The American Society for Dermatologic Surgery said the side effect also has been seen after viral and bacterial illnesses, other vaccinations and dental procedures. The group said people with dermal fillers should not delay or avoid the Covid vaccine. The side effect is rare, temporary and responds to treatments such as oral corticosteroids and an enzyme called hyaluronidase. The swelling also can resolve without treatment. The side effect has not been seen with wrinkle-relaxing injections like Botox or Dysport. If you’re concerned or not sure what type of injection you’ve gotten in the past, check with the doctor who gave you the cosmetic treatment. —Tara Parker-Pope

The Pfizer, Moderna, Johnson & Johnson and AstraZeneca vaccines don’t contain egg or any preservatives found in common vaccines. The vial stoppers are not made with natural rubber latex, so the vaccines are safe for people with latex allergy, according to the Allergy & Asthma Network. If you’ve ever had an allergic reaction to a vaccine, check with your doctor, but in most cases, you’ll still be encouraged to get the vaccine. As new Covid-19 vaccines come on the market, you should double check ingredient lists if you have had allergic reactions in the past. The Allergy & Asthma Network has published a chart showing all the ingredients in the Pfizer and Moderna vaccines.

The list of people who should not get vaccinated is very short. You should not get a second dose of the Pfizer or Moderna vaccine if you had a severe allergic reaction (anaphylaxis) to your first dose. Check with your doctor about whether you’re a candidate for another type of Covid vaccine when it becomes available. The Centers for Disease Control and Prevention also says you should not get the vaccine if you have a rare allergy to polyethylene glycol (PEG), a compound derived from petroleum and found in both vaccines. PEG is used in everyday products such as toothpaste and shampoo as “thickeners, solvents, softeners and moisture carriers,” and are also found in laxatives, according to Science magazine .You should not get the Pfizer, Moderna or Johhnson & Johnson vaccines if you are allergic to polysorbate, which is an ingredient in the Johnson & Johnson vaccine and closely related to PEG. Allergies to the ingredients are rare. —Tara Parker-Pope

No. The Pfizer, Moderna and Johnson & Johnson vaccines approved in the United States do not contain any live virus, weakened virus, dead virus or any infectious element, so there is no way for the vaccine to give you Covid-19. The best way to understand the mRNA vaccines is that they carry a set of instructions to teach your body’s immune system how to attack the coronavirus. The Johnson & Johnson vaccine uses a different technology to send similar instructions. —Tara Parker-Pope

Read More How 9 Covid-19 Vaccines Work

Two vaccines, Johnson & Johnson and AstraZeneca, have been linked to extremely rare clotting disorders. In the United States, federal health regulators have told Johnson & Johnson to add a warning to its label to note the potential risk of rare blood clots. European regulators who concluded that a vaccine made by AstraZeneca may also be the cause of a similar, extremely rare clotting disorder. In both cases, the risk is iinfinitesimal, and health officials have said the shot's benefits outwieigh the risk. The F.D.A. has not found a higher risk of clotting disorders in people who have received the Moderna or Pfizer-BioNTech vaccines. —Emily Anthes, Carl Zimmer and Noah Weiland

Read More The F.D.A. Ended Its Pause on the J&J Vaccine

Federal officials are reviewing hundreds of cases of rare heart problems following immunization with the coronavirus vaccines made by Pfizer-BioNTech and Moderna. The conditions are myocarditis, inflammation of the heart muscle, and pericarditis, inflammation of the membrane surrounding the heart. A study from Israel confirmed that the Pfizer-BioNTech Covid-19 vaccine is associated with an increased risk of myocarditis. But the side effect remains rare, and Covid-19 is more likely to cause myocarditis than the vaccine is, scientists say. —Emily Anthes, Noah Weiland and Apoorva Mandavilli

Read More Heart Problem More Common After Covid-19 Than After Vaccination, Study Finds

The C.D.C. has recommended that coronavirus vaccines be made available to pregnant women, though it also suggests that they consult with their doctors when making a decision about vaccination. Covid-19 poses serious risks during pregnancy. Pregnant women who develop symptoms of the disease are more likely to become seriously ill, and more likely to die, than nonpregnant women with symptoms.

In an early analysis of coronavirus vaccine safety data, C.D.C. researchers have found no evidence that the Pfizer-BioNTech or Moderna vaccines pose serious risks during pregnancy. The findings are preliminary and cover just the first 11 weeks of the U.S. vaccination program. But the study, which included self-reported data on more than 35,000 people who received one of the vaccines during or shortly before pregnancy, is the largest yet on the safety of the coronavirus vaccines in pregnant people. After vaccination, pregnant participants reported the same general pattern of side effects that nonpregnant ones did, the researchers found: pain at the injection site, fatigue, headaches and muscle pain. Women who were pregnant were slightly more likely to report injection site pain than women who were not, but less likely to report the other side effects. They were also slightly more likely to report nausea or vomiting after the second dose. —Emily Anthes

Read More No evidence that Pfizer or Moderna vaccines are unsafe during pregnancy, a preliminary study says.

A false claim has been circulating online that the new vaccine will threaten women’s fertility by harming the placenta. Here’s why it’s not true.

The claim stems from the fact that the vaccines from Pfizer and Moderna cause our immune systems to make antibodies to something called a “spike” protein on the coronavirus. The false claim about fertility risk is based on the unfounded concern that these antibodies could also attack a similar protein that is made in the placenta during pregnancy, called syncytin. In reality, the spike protein and syncytin are similar only in one very small region, and there’s no reason to believe antibodies that can grab onto spike proteins would lock onto syncytin.

What’s more, the human body generates its own supply of spike antibodies when it fights off the coronavirus, and there’s been no sign that these antibodies attack the placenta in pregnant women who become sick with Covid-19. If they did, you’d expect that women who got Covid-19 would suffer miscarriages. But a number of studies show that Covid-19 does not trigger miscarriages. —Carl Zimmer

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Obstetricians recommend being up-to-date on all vaccines before pregnancy, so it’s a good idea to get the Covid-19 vaccine as soon as you are eligible and can get an appointment. Covid-19 poses an especially high risk to pregnant women, so ideally you should get vaccinated before you become pregnant. “It’s really good if you can be protected from Covid before pregnancy,” said Dr. Denise Jamieson, an obstetrician at Emory University in Atlanta and a member of the American College of Obstetricians and Gynecologists committee on Covid vaccines.

The challenge for women of childbearing age is that most are not in the priority age groups scheduled to get the vaccine first. If the timing of your pregnancy does not matter to you, then it’s up to you if you prefer to get vaccinated first. But lack of access to the vaccine now should not be a reason to delay your pregnancy, experts say.

“Timing pregnancy is not necessarily as easy as we would like it to be, meaning it often takes women/couples time to conceive,” said Dr. Geeta Swamy, an obstetrician at Duke University in Durham, N.C., and a member of the ACOG vaccine committee. “Additionally, it is unclear when nonpregnant women will be eligible for vaccination if they are not in a high-risk category to get vaccinated. So given all of these unpredictable aspects combined with the fact that we have no concerns about vaccination impacting conception/early pregnancy, ACOG and others do not recommend delaying pregnancy until after vaccination.” —Dani Blum

Fertility patients who are scheduled for procedures like egg retrieval, embryo transfer or intrauterine insemination are advised to avoid getting a Covid vaccine within three days before and three days after the procedure, according to the American Society for Reproductive Medicine. That’s because patients undergoing surgical procedures could develop vaccine-related side effects like fever or chills that might make it difficult for doctors to know if a post-surgical infection is brewing. In addition, many medical providers may not allow a patient who is experiencing Covid-like symptoms into their facility, even if it’s likely that the symptoms are from a vaccine and their Covid-19 test is negative. —Christina Caron

Read More What Women Need to Know About the Covid Vaccine

Coronavirus vaccinations can cause enlarged lymph nodes in the armpit that will show up as white blobs on mammograms. This type of swelling is a normal reaction to the vaccine and will typically occur on the same side as the arm where the shot was given, said Dr. Geeta Swamy, a maternal-fetal medicine specialist and a member of the American College of Obstetricians and Gynecologists’s Covid vaccine group. It usually only lasts for a few weeks. But the vaccine’s effect on mammograms can be concerning to radiologists, she added, because “if someone had breast cancer we might see enlarged lymph nodes as well.” Because this type of swelling could be mistaken as a sign of cancer, the Society of Breast Imaging recommends trying to schedule your routine mammogram before your first Covid-19 vaccine dose or at least one month after your second vaccine dose. The guidance is only for women getting routine mammograms. If you are getting a mammogram because of a suspicious lump or other symptoms, don’t delay. You should keep your current mammogram appointment as well as your vaccination appointment, and tell your radiologist the date that you received the vaccine. —Christina Caron

Read More What Women Need to Know About the Covid Vaccine

Some women say they have observed changes in the flow or timing of their period after getting vaccinated. So far, there’s no data linking the vaccines to changes in menstruation. Even if there is a connection, one unusual period is no cause for alarm. There is a long list of triggers that can cause changes to the menstrual cycle, including stress, illness and changes in diet and physical activity. Although more study is needed, there is a link between menstruation and the immune system. Both the thickening and thinning of the uterine lining are facilitated by different teams of immune cells and signals moving in and out of the reproductive tract; one wave helps to build, others help to dismantle. The process of shedding this lining during menstruation is in part an inflammatory response, which is why women often experience cramping and pain during this stage.

Since the cycle is supported by the immune system at every turn, it is possible that the vaccines, which are designed to ignite an immune response, could temporarily change the normal course of events. For example, an activated immune system might interfere with the usual balance of immune cells and molecules in the uterus. These types of disturbances have been found in studies to contribute to changes in periods, including heavy menstrual flows. But no one can say whether this may explain potential post-vaccine disruptions to the menstrual cycle. To find out, we would need a controlled study with a placebo group. Clinical trials, including those for vaccines, typically omit the tracking of menstrual cycles, so we lack the evidence required to put these reports in context. If you have questions about your menstrual cycle, be sure to speak with your doctor. —Alice Lu-Culligan and Randi Hutter Epstein

Read More No, We Don’t Know if Vaccines Change Your Period

Even if you’ve had Covid-19, you still will get stronger immunity from vaccination. A person’s immune response to a natural infection is highly variable. Some people may produce few antibodies, and some variants seem to dodge natural antibodies more easily than stronger vaccine-generated antibodies. While it’s not clear how much extra benefit a recovered Covid patient gets from two doses, versus a single dose, you need a second dose to provide proof of full vaccination, should you need it for travel or for work. People who have had Covid-19 in the past are advised to wait about 90 days after infection before getting vaccinated if they were treated with convalescent plasma or monoclonal antibodies. If you get Covid-19 after your first dose, you may need to adjust your vaccination schedule until you are fully recovered and no longer need to isolate. Check with your doctor about the best timing if you’re not sure. —Tara Parker-Pope

Read More ‘Natural Immunity’ From Covid Is Not Safer Than a Vaccine

People with weakened immune systems don't generate as robust a response to the vaccines as others. The C.D.C. has advised people with weakened immune systems to get a third dose of the Pfizer or Moderna vaccine four weeks after the second shot. This extra shot is not technically a booster shot, but is now considered part of the regular immunication schedule for this group of patients. About 3 percent of Americans have weakened immune systems for a variety of reasons, including organ transplants, a history of cancer or use of medications like steroids that suppress the immune system. —Noah Weiland and Sharon LaFraniere

Read More The F.D.A. authorized a third dose of Covid vaccines for immunocompromised people.

Patients in active treatment should consult with their medical team about how and when to get the vaccine. Some patients may be advised to time the vaccine, if possible, between rounds of chemo — when their white blood cell counts are highest — to optimize their immune response. Ideally, cancer patients in active treatment should receive vaccinations under the care of a doctor, or in a cancer center, where they can be closely monitored and are likely to encounter fewer people than they would at a mass distribution site. The state-by-state nature of vaccination rules can complicate vaccination for cancer patients. For instance, if you live in one state and get cancer treatment in another, your cancer center may not be allowed to give the vaccine to an out-of-state patient.

Patients with a history of cancer or undergoing active treatment may be advised to follow the three-dose immunization schedule recommended for patients with weakened immune systems. —Dani Blum

Read More In the Vaccine Scramble, Cancer Patients Are Left Behind

An estimated 8% of Americans have an autoimmune disease such as rheumatoid arthritis or lupus, which occur when the immune system mistakenly attacks normal body tissues. Although people with autoimmune conditions were allowed to enroll in the Covid vaccine trials, patients were excluded if they were taking the type of immune-suppressing drugs used to treat autoimmune disorders. As a result, doctors believe the vaccine is safe for people with autoimmune conditions, but they don’t have specific data showing how well the vaccine works in these patients. The National Institutes of Health has announced a new study to determine whether people with autoimmune conditions should receive a third dose of Covid vaccine, similar to the immunization schedule approved for patients with weakened immune systems. The study will also investigate whether pausing immunosuppressive therapy for autoimmune disease improves the antibody response to an extra dose of a COVID-19 vaccine. In the meantime, patients with autoimmune conditions should consult with their physicians about getting the Covid vaccine. —Dani Blum

While there’s not yet a lot of data about this scenario, vaccine researchers say there’s no cause for additional worry if you find out you were infected at the time of vaccination. “You won’t feel very well, but that’s due to the Covid, not the Covid vaccine,” said Dr. Helen Talbot, a member of a panel advising the Centers for Disease Control and Prevention and an infectious disease specialist at Vanderbilt University.

It’s unlikely the first dose of vaccine has had enough time to help your body fight the infection, but the vaccine should still spur your body to produce a lasting immune response. Once you’ve recovered, you should plan to get your second dose as planned. “If you unknowingly have the virus and are immunized, the vaccine will not prevent disease but will likely help in the overall development of immune response,” said Dr. Talbot. “You would then get your second immunization once fully recovered — likely after the usual 21 or 28 days. No need to start the vaccine series over." —Dani Blum

Guillain-Barré syndrome is a rare and serious condition that occurs when the body’s immune system attacks the nerves, causing muscle weakness and sometimes paralysis. Nobody knows exactly what causes it, but most patients report that they had recently recovered from a respiratory or gastrointestinal infection. Guillain-Barré syndrome also has been pinpointed as a possible complication of Covid-19.

Although regulators have found that the chances of developing the condition are low, they appear to be three to five times higher among recipients of the Johnson & Johnson vaccine than among the general population in the United States. The F.D.A. has now attached a warning to the Johnson & Johnson shot about the increased risk of developing Guillain-Barré after vaccination.

For people who have had Guillain-Barré in the past, the guidance has been confusing. The official guidance from the Centers for Disease Control and Prevention is that people with a past history of Guillain-Barré should get the Covid vaccine. But Dr. Anthony S. Fauci, the nation’s leading infectious disease expert, has said that he does not recommend that people with a history of Guillain-Barré get the vaccine. “We recommend those people do not get vaccinated because you might trigger a similar serious response,” Dr. Fauci said.

But not everybody agrees with that advice. Members of the GBS/CIDP International Foundation, an advocacy group for people with Guillain-Barré, wrote an open letter to Dr. Fauci urging people with the syndrome to still get the vaccine. “At this time, there is no reason that those who had GBS in the past cannot get the current Covid vaccines,” they wrote. “If they have concerns, they should speak to their local health care professionals.” —Dani Blum

Read More F.D.A. Attaches Warning of Rare Nerve Syndrome to Johnson & Johnson Vaccine

Trials studying the Covid-19 vaccine have included people with H.I.V., albeit in small numbers. The Pfizer trial included 120 people with H.I.V., and the Moderna trial included 176 people with H.I.V., according to Poz , a news site for people living with and those affected by H.I.V./AIDS. Although the numbers are too small to draw meaningful conclusions, no unusual safety concerns were reported for people with H.I.V. Because the Pfizer and Moderna vaccines do not contain weakened or inactivated virus, they are believed to be a safe option for people with H.I.V. and AIDS. Like other people with weakened immune systems, people with H.I.V. and AIDS who received the Pfizer or Moderna vaccines now are eligible for a third shot four weeks after the second dose. —Tara Parker-Pope

Although these are described as “genetic” vaccines, the new Covid vaccines don’t alter your genes in any way. Instead, the Pfizer-BioNTech and Moderna vaccines use a genetic molecule to prime the immune system. That molecule is known as mRNA — the “m” stands for messenger. Think of this molecule like a set of instructions. While a traditional vaccine uses a weakened or inactivated germ to trigger an immune response in our bodies, the mRNA vaccines carry a set of instructions to teach our cells how to make a protein that will trigger an immune response and produce antibodies to the virus.

Here’s how it works. When you get your shot in the arm, the injection includes the messenger molecule, which is packaged in an oily bubble that fuses to a cell. The cell then uses the mRNA molecule as a set of instructions to make something called a “spike protein.” (The surface of the coronavirus is covered with similar spikes.)

Your immune system quickly recognizes that the spike protein is a foreign invader, and begins attacking it. The vaccine has essentially trained your immune system to recognize and attack the spike. Now, if you ever come into contact with the actual coronavirus, your immune system has learned how to handle it. You can learn more about mRNA vaccines from the Centers for Disease Control and Prevention website. —Tara Parker-Pope

Read More How the Pfizer-BioNTech Vaccine Works

No! The messenger RNA used in the in the Pfizer and Moderna vaccines never enters the nucleus of your cells and has no effect on your DNA. The mRNA molecule survives long enough for your cells to copy the instructions and create the spike protein that is used to train your immune system against coronavirus. So what happens to the molecule after it delivers these instructions? It lasts a few days, and then the cell shreds it and gets rid of it using special enzymes. It’s important to understand that the vaccine mimics a natural process that goes on in your body every day. At any moment, each of our cells may contain hundreds of thousands of mRNA molecules, which they produce in order to make a wide variety of proteins of their own. —Tara Parker-Pope and Carl Zimmer

Like the Pfizer and Moderna vaccines, the Johnson & Johnson vaccine gives the body a set of instructions to build a spike protein that can train the immune system to ward off a coronavirus infection. While the Pfizer and Moderna vaccines use a genetic molecule called mRNA to provide the instructions, the Johnson & Johnson vaccine uses DNA to give the message, and the DNA is carried by a so-called viral vector, Adenovirus 26. Adenoviruses are common viruses that typically cause colds or flu-like symptoms. The Johnson & Johnson team used a modified adenovirus that can enter cells to deliver the instructions, but can’t replicate inside them or cause illness.

Johnson & Johnson’s vaccine comes out of decades of research on adenovirus-based vaccines. In July, the first one was approved for general use — a vaccine for Ebola, also made by Johnson & Johnson. The company is also running trials on adenovirus-based vaccines for other diseases, including H.I.V. and Zika. Some other coronavirus vaccines are also based on adenoviruses, such as the one developed by the University of Oxford and AstraZeneca, using a chimpanzee adenovirus.

Adenovirus-based vaccines for Covid-19 are more rugged than mRNA vaccines from Pfizer and Moderna. DNA is not as fragile as RNA, and the adenovirus’s tough protein coat helps protect the genetic material inside. As a result, the Johnson & Johnson vaccine can be stored using traditional refrigeration methods for up to three months. —Carl Zimmer and Tara Parker-Pope

Read More How the Johnson & Johnson Vaccine Works

While more than 70 countries have authorized the vaccine, the United States has not. AstraZeneca has not yet applied to the Food and Drug Administration for authorization. —Denise Grady and Rebecca Robbins

Covid-19 is by far the more dangerous option. Covid vaccines carry little known risk. But the perils of Covid-19 have been well documented. About 20 percent of people who come down with Covid-19 symptoms develop serious, potentially life-threatening illness. Although people who are older, obese or have other health problems are at highest risk for complications from Covid-19, younger people can become severely ill, too. In a study of more than 3,000 people ages 18 to 34 who were hospitalized for Covid, 20 percent required intensive care and 3 percent died.

The long-term health complications associated with Covid-19 are a serious concern. As many as one in three people who recover from Covid have chronic complaints for months afterward, including exhaustion, a racing heart, blood clots and loss of sense of smell or taste. —Apoorva Mandavilli

Read More ‘Natural Immunity’ From Covid Is Not Safer Than a Vaccine

While the rise of more infectious variants, including the highly-infectious Delta variant, has caused cases of Covid-19 to surge around the world, the risk is primarily to the unvaccinated, for whom there is great concern. But for the vaccinated, the outlook is much more hopeful. While it’s true that the vaccines have different success rates against different variants, the perception that they don’t work against variants at all is incorrect. In fact, the available vaccines have worked remarkably well at preventing serious illness and hospitalization, even as new variants circulate around the globe.

The variants are “all the more reason to get vaccinated,” said Dr. Anthony S. Fauci, the nation’s top infectious disease specialist. “The bottom line is the vaccines we are using very well protect against the most dominant variant we have right now, and to varying degrees protect against serious disease among several of the other variants.” —Tara Parker-Pope

Read More Can the Covid Vaccine Protect Me Against Virus Variants?

The false conspiracy theory about microchips emerged after Bill Gates, the founder of Microsoft, made a comment about “digital certificates” that might one day be used to show a person had been tested or vaccinated for Covid-19. The reference prompted conspiracy theories to circulate online speculating that a tracking microchip would be planted by the government to surveil the movements of Americans. For months, widely shared videos and viral posts on social media have baselessly claimed that such technologies could find their way into syringes delivering shots. None of the rumors are true.

The Pfizer and Moderna vaccines have one active ingredient: a molecule called messenger RNA, or mRNA, which contains genetic instructions for a coronavirus protein called spike. Once injected, the mRNA will instruct human cells to manufacture spike, exposing the immune system to a highly recognizable feature of the virus. The remaining ingredients are lipids, including cholesterol, that form a fatty bubble around the fragile mRNA molecule, as well as sucrose (sugar) and various salts. —Katherine J. Wu and Tara Parker-Pope

Read More No, There Are No Microchips in Coronavirus Vaccines

Of the three vaccines approved for use in the United States, only the Pfizer vaccine has received formal approval from the F.D.A. The other two vaccines -- Moderna and Johnson & Johnson -- have been authorized for emergency use. An emergency use authorization is surprisingly similar to a standard approval. It includes the same basic steps — preclinical testing, Phase 1 safety trials, Phase 2 expanded trials and Phase 3 efficacy trials — that would be required in the traditional approval process. The main difference is that, in an emergency, the F.D.A. gives the application priority and speeds up its own review of the research. One way to do that is to solve logistical concerns early, while waiting for clinical trials to finish. For instance, the F.D.A. worked with the vaccine companies to solve manufacturing and distribution issues before the firms had completed their clinical trials or submitted applications for emergency use. In an interview with Scientific American magazine, the former F.D.A. commissioner Dr. Stephen Hahn said that in an emergency, the agency can prioritize an application over other demands to reduce the four-to-six-month review process to just several weeks.

Once vaccine makers win an emergency use authorization, they continue collecting information on the safety and efficacy of the vaccine and apply for a license, which is the final step in the approval process. Additional data will be collected on special patient populations like children, pregnant women and immune-compromised patients who weren’t studied in the first round of research. —Tara Parker-Pope and Carl Zimmer

Read More The Vaccine Testing Process

Vaccination efforts against Covid-19 have revealed an extraordinary gap in access to the vaccines around the world. Rich nations like the United States and Britain have cut deals with multiple drug manufacturers and secured enough doses of vaccines likely to come on the market this year to immunize their citizens multiple times over. China and Russia have developed their own vaccines and begun mass immunization programs. In stark contrast, most poor nations rely on a complex global vaccine-sharing initiative called Covax, and are likely to receive only enough doses to vaccinate at most 25 percent of their populations this year. Run by the World Health Organization and two global nonprofits, Covax relies on financial assistance and other support from wealthy nations. It wasn’t until this month that the U.S. agreed to participate and provide funding.

Worldwide, 81 percent of shots that have been administered have been in high- and upper-middle-income countries, according to the Our World in Data project at the University of Oxford. Only 0.4 percent of doses have been administered in low-income countries. —Megan Twohey, Keith Collins and Katie Thomas

Read More Rich countries have first dibs on vaccines, while poor nations struggle to get enough.

The U.S. Centers for Disease Control has said a person is fully vaccinated two weeks after receiving the final dose of whatever vaccine you’ve been given. If your vaccine requires only one dose, it will take about two weeks for your body to build a strong immune response. If your vaccine requires two doses, the peak vaccine response won’t be reached until two weeks after the second dose. —Carl Zimmer and Denise Grady

Read More Pfizer’s Vaccine Offers Strong Protection After First Dose

There’s no one-size-fits-all answer to those questions because risk changes from one individual to the next, depending on a person’s overall health, where they live and those they spend time with. While most people who get vaccinated will not experience a breakthrough infection, some people will. Taking reasonable precautions, like wearing a mask in public indoor spaces when you don't know the vaccination status of others and avoiding crowded gatherings, will lower your risk. The vaccines remain highly protective against serious illness. A recent study in Los Angeles County showed that while breakthrough infections can happen, the unvaccinated are 29 times as likely to end up hospitalized from Covid-19 as a vaccinated person. —Tara Parker-Pope

Read More What Vaccinated People Need to Know About Breakthrough Infections

While unvaccinated people are by far at highest risk for catching and spreading Covid-19, it’s also possible for a vaccinated person to become infected and transmit the illness to others. A recent outbreak in Provincetown, Mass., where thousands of people gathered in bars and restaurants, showed that vaccinated people can sometimes spread the virus.Even so, many experts believe the risk of getting infected from a vaccinated person is still relatively low. A study from Singapore looked at vaccinated and unvaccinated people infected with the Delta variant. The researchers found that while viral loads in vaccinated and unvaccinated workers are similar at the onset of illness, the amount of virus declines more rapidly in the vaccinated after the first week, suggesting vaccinated people are infectious for a shorter period of time. —Tara Parker-Pope

If your vaccine requires two doses, you're not fully protected against Covid-19 while you’re waiting for your second shot. There have been a few reports of people appearing to become infected with the virus after receiving their first shot. In most cases, it’s not known whether the patients already had the virus when they were given the vaccine, or whether they were exposed after vaccination but before their bodies had built up enough immunity to fight off the virus. You should still plan on getting the second dose on schedule, but check with your doctor first. And remember, even after two doses, no vaccine offers 100 percent protection. But even if you do catch the virus after vaccination, it's likely that you will experience mild illness because your body, has antibodies ready to fight off the virus. —Katherine J. Wu

While being fully vaccinated protects against serious illness and hospitalization from Covid-19, no vaccine offers 100 percent protection. As long as large numbers of people remain unvaccinated and continue to spread coronavirus, vaccinated people will be exposed to the Delta variant, and a small percentage of them will develop so-called breakthrough infections.

It's a good idea to wear a mask when you don't know the vaccination status of those around you, particularly when you're in indoor public spaces. The risk is lower if you live in a community where vaccination rates are high and overall case rates are low and dropping. Most public health experts agree it's safe for fully vaccinated people to spend time indoors and unmasked with vaccinated friends and family members. But vaccinated people who have been traveling or recently attended a large gathering may want to get tested or avoid spending time unmasked with others until they are sure they weren't infected. —Tara Parker-Pope

Read More Should Vaccinated People Start Wearing Masks Again?

Employers do have the right to compel their workers to be vaccinated once a vaccine is formally approved. Many hospital systems, for example, require annual flu shots. But employees can seek exemptions based on medical reasons or religious beliefs. In such cases, employers are supposed to provide a “reasonable accommodation” — with a coronavirus vaccine, for example, a worker might be allowed to work if they wear a mask, or to work from home.

The Biden administration has mandated that all companies with more than 100 workers require vaccination or weekly testing. Mr. Biden also moved to mandate shots for health care workers, federal contractors and the vast majority of federal workers, who could face disciplinary measures if they refuse. —Abby Goodnough

Read More Employers Can Require Workers to Get Covid-19 Vaccine, U.S. Says

Some countries now require travellers to provide proof of vaccination as well as a negative Covid test before being allowed to board flights. Before traveling internationally, be sure to check the U.S. State Department’s detailed COVID-19 travel information at Travel.State.gov because travel restrictions can change quickly. And beginning in early November, all adult foreign nationals will be required to be fully vaccinated and show proof of vaccination against COVID-19 prior to boarding flights to the United States. —Sarah Firshein

Read More What Will I Need to Show When Traveling and Where?

If you got your shot at a pharmacy, an employee can print out a new card from your electronic records. Vaccinations are also tracked by state health departments, so you can reach out to your state’s agency to get a replacement card.

Read More What You Need to Know About Your Covid-19 Vaccine Card