Signs point to COVID-19 vaccines slowing viral transmission, not just disease

Rochelle Kern receives her first COVID vaccine .

Researchers say recent findings bode well for vaccines helping slow the spread of the pandemic

By Jonathan WosenBiotech reporter

Feb. 27, 2021 5 AM PT

After countless hours spent searching for a COVID-19 vaccine appointment, Elizabeth Kostas is days away from getting her second and final shot. And like many San Diegans, she’s wondering what she’ll be able to do next.

“I just look back longingly and with such good memories of hanging out with friends and doing fun things,” said the 69-year-old Carmel Valley resident. “I want to get back to that.”

But Kostas plans to keep playing it safe for now, and will continue to wear a mask, stay at home and avoid traveling. She’s worried about unwittingly spreading the virus to family, friends and neighbors who haven’t gotten their shot yet.

There’s still plenty of confusion and concern around whether coronavirus vaccines protect you from infecting others, but a recent slate of studies suggests that they do.

Scientists stress that the benefits are partial, and that vaccinated people shouldn’t throw caution (or their masks) to the wind. But they also say the information can help people who’ve been vaccinated make safe and sensible decisions about getting together with friends and family — especially if they’ve also been immunized.

“Everyone that’s around a vaccinated person is now getting a benefit (because) that person has a much lower chance of being infected and being able to then pass it on,” said Marm Kilpatrick, an infectious disease researcher at UC Santa Cruz. “That is, I think, the most valuable impact.”

The simplest way to show that COVID-19 vaccines reduce transmission is to show they lower a person’s odds of carrying the virus. After all, you can’t spread an infection that you don’t have.

A study posted online on Feb. 22 by U.K. researchers shows exactly that. Scientists tracked more than 23,000 health care workers over two months and tested them for the coronavirus every two weeks, regardless of whether they felt sick.

By comparing health care workers who had or hadn’t been immunized, researchers found that a single COVID-19 vaccine dose was 70 percent effective in reducing infections, and that two doses of the Pfizer or AstraZeneca vaccines were 85 percent effective. Those numbers reflect a decline in total infections — both those that cause COVID-19 symptoms and silent, asymptomatic infections that can spread from person to person.

Even when vaccines don’t prevent infection, they can reduce the amount of virus that you carry. That’s what Israeli researchers reported in early February after analyzing positive coronavirus test results; Israelis who had been vaccinated had an estimated 3 to 4.7 times less virus than those who hadn’t.

That’s helpful considering that having less virus generally makes you less infectious. Spanish researchers tracked more than 300 people with COVID-19 and their close contacts. Those who had lower levels of the coronavirus were less likely to infect family, friends and others.

None of these findings are shocking, says UCSC’s Kilpatrick. But they are reassuring.

“All of us that work on this kind of stuff for a living were hoping we’d get exactly this outcome.”

Kilpatrick has studied the spread of West Nile virus and Lyme disease, among other infections, and his own best estimate is that the current COVID-19 vaccines reduce viral transmission by 90 percent.

Tracking COVID-19 transmissions amid vaccinations

That estimate is baked into the models UC San Diego’s Natasha Martin uses to forecast the spread of the pandemic here in San Diego County. The working assumption is that, for every 10 San Diegans exposed to the coronavirus after being vaccinated, only one of them will spread the virus to others.

That’s consistent with the limited local data available. According to Dr. Eric McDonald, the county’s chief epidemiologist, contact tracing efforts haven’t found any San Diegans who tested positive for the coronavirus because they were likely infected by someone who’d been fully vaccinated.

It’s too early to draw firm conclusions from this data, however, given how few San Diegans have gotten the first and second vaccine doses needed to maximize immunity against the coronavirus. And scientists are still learning more about how long immune responses against the coronavirus last, as well as how well vaccines limit the spread of new, faster-spreading viral variants. Still, current evidence all points in the right direction.

The lingering uncertainty around vaccines’ effects on transmission stems from the COVID-19 vaccine trials being set up to focus on whether the vaccines prevented people from getting sick.

“We decided to go as fast as we can to stop severe infection versus to tease out the answer around transmission and asymptomatic infection,” said Dr. Davey Smith, UCSD’s chief of infectious disease research.

Illness is easier to detect than transmission, and by that measure, the vaccines from Pfizer and Moderna are about 95 percent effective. But around 1 out of every 5 people infected by the coronavirus may never develop symptoms, according to an analysis of current infection data, though they can still spread the virus to others.

Some drugmakers have generated data suggesting their vaccines also limit transmission. U.K. participants in AstraZeneca’s vaccine trial swabbed their own noses weekly, which showed that the vaccine reduced total infections by more than 50 percent. And when Moderna applied for emergency approval of its vaccine in the U.S., it noted that, of the 52 trial participants who had asymptomatic infections before getting their second shot, 38 were in the placebo group compared to 14 in the vaccine group.

Smith is leading a national study that he hopes will go one step further — directly showing that vaccines reduce transmission by collecting data on the household contacts of people who’ve been immunized.

“You’re most likely to transmit to the people you love, and the people you live with,” Smith said.

His team plans to measure antibody levels in vaccinated participants. Their working hypothesis is that people living with someone who has mounted a strong immune response to the vaccine should be less likely to get infected.

Kostas sure hopes that’s the case.

“That’s music to my ears,” she said. “If that comes to be accepted as true, that would be a game changer.”

What you need to know about San Diego's COVID-19 vaccine rollout

UPDATED May 5, 2021 | 9:16 AM

Am I eligible for the vaccine?

Anyone 16 and up is eligible.

Where can I get my shot?

The region has mass vaccination sites in La Mesa, La Jolla, Chula Vista, San Marcos and Del Mar. There are also many smaller sites throughout San Diego County, some run by Sharp HealthCare and Rady Children’s Hospital. The county’s COVID-19 vaccine page lists the various locations and ways to sign up; we have a similar resource on the U-T’s website. Local pharmacies are an option, too. CVS is now offering walk-in (appointment-free) vaccination as well as same-day appointments. If you have a Facebook account, you may want to join the San Diego Vaccine Hunters group, where more than 6,000 people ask and answer questions about how to book an appointment.

If you want to make an appointment at one of the smaller county-run sites, you can do so through an online system called MyTurn (myturn.ca.gov). Several of these sites are now also accepting walk-ins.

County officials have said that seniors who don’t have a computer can call 2-1-1 for help scheduling vaccination appointments. There’s also a group called San Diego Vaccine Angels, which is helping seniors and those who aren’t tech savvy book appointments.

Health care providers such as UC San Diego Health, the San Diego VA, Sharp and Scripps have been reaching out to older patients within their systems to schedule appointments, but that is happening slowly and unevenly between providers depending on how much vaccine they have.

K-12 teachers and staff won’t have to schedule their own vaccine appointments, as that will be handled through a program run by the San Diego County Office of Education and California Schools Voluntary Employees Benefits Association (VEBA), an organization that offers health care benefits to school employees. Scripps Health will work with law enforcement agencies to ensure that officer are immunized and San Diego firefighters will go out into the fields to vaccinate farm workers.

Can I pick which vaccine I get?

Sort of. Some of the the county’s vaccine locations only offer the Moderna vaccine. Others offer the Pfizer vaccine, such as the Del Mar Fairgrounds superstation.

But both of those vaccines use the same approach to spark immunity against the coronavirus and are about 95 percent effective at preventing you from getting sick with COVID-19.

Speaking of which, you’ll need to come back for a booster shot three or four weeks later for Pfizer’s and Moderna’s vaccines, respectively. That second dose maximizes immunity against the virus. When you get your first shot, you’ll receive a vaccination record card reminding you to return for that next appointment.

The most recently authorized vaccine, made by Johnson & Johnson, only requires one shot.

How much does the shot cost?

The shot is free — though, of course, the U.S. government used taxpayer dollars to purchase the doses. Vaccine providers can bill your insurance an administration fee, but there’s no copay. And if you don’t have insurance, you’ll still get the vaccine at no cost.

What can I do after getting vaccinated?

The Centers for Disease Control and Prevention say it’s OK to get together with small numbers of other vaccinated people indoors — with or without a mask — once you’ve been fully vaccinated (e.g. a dinner party). If you received the Pfizer or Moderna vaccine, that would be two weeks after your second dose. If you received the Johnson & Johnson vaccine, that would be two weeks after your shot.

The CDC says it’s also OK for fully vaccinated people to gather with unvaccinated people who live together, as long as those people are at low risk of developing severe COVID-19. And the agency has said that fully vaccinated people can travel within the U.S. without getting a coronavirus test before or after their trip or needing to quarantine after their travels. Those who travel internationally won’t need a negative coronavirus test unless it’s required by the nation they’re traveling to, but will still need a negative test result before boarding a flight back to the U.S. and must get another test within three to five days of returning from their trip.

Caution is still key. The CDC says to avoid medium- or large-sized gatherings regardless of vaccination status, and to wear a mask during large outdoor gatherings. Part of the issue is that you don’t have to have COVID-19 symptoms to spread the coronavirus, and many people still haven’t been vaccinated yet. Researchers say that throwing caution to the wind could further fuel the spread of the virus and even lead to new strains that vaccines don’t work against — undermining the whole immunization campaign.