Covid-19 has established itself as the deadliest, fastest-moving pandemic since 1918, and is nearing a half-million fatalities. During the century that separates the two outbreaks, the world battled a string of other devastating diseases including Ebola, SARS and several other flu pandemics.
Though past outbreaks can’t answer questions about Covid-19 such as when a licensed vaccine might become available, they can offer important insight into how diseases spread and sometimes resurge in second waves. The novel coronavirus is continuing to spread across the globe, igniting new hotspots with as many as 150,000 new cases every day. Over six months into the global crisis, more than 9 million people have been infected, a number that’s certainly an undercount given how few people are being tested in places like Brazil and Mexico, the newest epicenters.
Meanwhile cases are spiraling up again in the U.S., long the leader in total infections and deaths and now experiencing increasing cases across southern and western states.
Pandemics like Covid-19, although rare, have been observed closely and have a number of common characteristics that separate them from other outbreaks of new infectious diseases that may result in thousands—not millions—of cases and can be readily extinguished with public health and medical tools. While measures like testing, tracking and tracing, and social distancing can help control their spread, the stealthiness and infectiousness of these pathogens make them difficult to control. Any case that goes unaccounted for has the possibility of starting a new flare-up.
Outbreaks of infectious disease manifest in many different shapes and sizes. Some, like malaria, continue to spread widely for centuries, even in the face of effective means of control. Others, like SARS, arise quickly and are tamed in just a few months. After causing millions of cases of death and disfigurement, smallpox was eradicated with vaccination; polio appears headed in the same direction.
While severe acute respiratory syndrome, or SARS, spread around the world after originating in China, it’s an example of an outbreak that never reached pandemic status. Like the new coronavirus, it was relatively stealthy at first, with cases of mysterious pneumonia cropping up as early as November, 2002. But SARS never really took off in the community.
Clusters of disease outside of China—in Toronto, Singapore and other cities—were localized around hospitals. Health-care workers were highly vulnerable to the virus as they cleaned out patients’ lungs and performed other tasks of care. The disease was stamped out by intense tracking, tracing and isolation of infected people and their contacts, along with travel restrictions that kept it from being exported to additional cities and patients.
Researchers began working on vaccines against SARS, but by the time they were ready to be tested, the disease had been controlled, and was later eliminated. China’s delayed reaction to the disease may have inspired the government to respond to the new coronavirus more quickly and openly, although it appears there was initial resistance to acknowledging its presence.
Influenza viruses are known to regularly mutate and evolve into new forms that pose an ongoing challenge to the immune system. Sometimes the new form is sufficiently different from existing flu viruses that it’s unrecognizable to the body’s defenses, and can quickly spread and cause severe disease.
The 2009 influenza pandemic was first seen in April of that year and rapidly spread around the world: 74 countries had cases by June, according to the World Health Organization. Most flu and respiratory viruses tend to fade in warm weather, moving to the southern hemisphere when the north is hot and vice versa. Pandemic flu, however, often spreads in warm months, possibly because it has extra transmission power due to the lack of immune opposition.
While seasonal flu normally affects older people especially, the 2009 influenza was an H1N1 strain—a relation to the 1918 pandemic and consequently one that many older people had some immune protection against. In fact, much of the mortality from this pandemic occurred in younger adults and people who were otherwise healthy.
The 1918 influenza pandemic, sometimes called the Spanish flu, is the world’s most deadly in recent memory. It probably originated in the U.S., not Spain, during the spring of that year and possibly was first transmitted to people from birds. Its rapid spread was probably helped by World War 1 soldiers who amplified the epidemic in crowded training camps and were then deployed to Europe, where living conditions were just as cramped and fatalities were common. While 16 million perished in the war, some 50 million died from the flu, many of them in a second wave of infections, also first detected in the U.S. in the latter part of the year.
In some parts of the world, this second wave was even more deadly than the first, possibly because of mutations in the virus. Coronaviruses, on the other hand, appear to be less prone to mutation than flu, possibly giving vaccine makers a steadier target to aim at.
The 1968-69 influenza pandemic, sometimes called the Hong Kong flu because of where it was first observed, also had a noticeable second wave. Researcher Cecile Viboud and others suggest that the first pandemic season was more deadly in the U.S., while the second wave caused more mortality in Europe and Asia. This may have been due to genetic changes in the virus along with patterns of immunity in local populations, according to Viboud, who’s a researcher at the U.S. National Institute of Health’s Fogarty International Center.
One of the few things that we may be able to take with us from this pandemic, and will certainly be studied by future researchers, is the quick implementation of lockdowns and social distancing. Hundreds of millions of people worldwide have been confined to their homes and avoided contact with one another in an effort to keep the virus from spreading. Without these measures, the ascent of cases would likely have been much steeper, and perhaps still rising. The quick delivery of vaccines, perhaps in less than a year, is another possible feature of the Covid-19 outbreak, should researchers develop one that’s safe and effective.
Update: The estimate of global deaths from H1N1 has been updated.