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After COVID, will the flu make a comeback?

The first cold and flu season after the pandemic could be tough. But we can apply the lessons we've learned.

By Veronique Greenwood

In the early months of the COVID-19 pandemic, a curious trend emerged at Connecticut’s Stamford Hospital. As the state took measures now known to be effective in curbing the coronavirus — limiting gatherings, closing schools and businesses, and mandating masks — cases of flu and other respiratory viruses took a nosedive.

“We were surprised,” public health scientists at the hospital wrote in a paper documenting the decrease. Only 1.3 percent of flu tests were positive, compared with 10.8 percent at the same time the prior year — despite far higher numbers of tests.

Such serious declines in the flu and other illnesses have been welcome side effects as hospitals struggle with the load of the pandemic. But that relief will be short-lived, researchers warn. As vaccinations increase and people begin to gather again, some viral infections may not only bounce back, but roar back, exceeding the levels we’re used to. And thanks to an effect familiar to epidemiologists, the problem may appear a year or more after the pandemic is over, when we’ve let down our guard.

That’s a danger, but also an opportunity, researchers say — to fundamentally rethink the way we live with viruses, including those less dangerous than COVID-19. Viruses surround us, for better or for worse, and we can be better prepared — by changing our behavior, boosting vaccination rates, and tracking our immunity to milder viruses with far more rigor than we have before.

Through the COVID ordeal, “we have seen that there is a playbook for stopping epidemics” — involving mask-wearing, investment in ventilation technologies, and more robust paid sick leave policies, says Samuel Scarpino, an assistant professor at Northeastern University’s Network Science Institute. “Investment and planning now will be essential for a more normal life going forward.”

Humans and viruses are engaged in a constant, mostly invisible dance of infection, recovery, and immunity. You get sick, you get better, and for some diseases, you’re protected from getting the virus again for a limited period of time. As people’s immunity waxes and wanes, it can shape the levels of these diseases in the population.  

Some viral infections may not only bounce back, but roar back.

Rachel Baker, a disease modeler at Princeton University, studies the seasonal dynamics of respiratory syncytial virus (RSV), which hits most people like the common cold but can cause dangerous illness in young children. Researchers have noticed that in some places, like northern Minnesota, infection rates vary greatly because of these tides of immunity. If an RSV outbreak is particularly large one year, the next year’s will be smaller, because more of the population is immune. “That cycle repeats itself year after year,” Baker says.

Baker was a co-author of a paper, published in the Proceedings of the National Academy of Sciences, that brings those ideas to bear on how those peaks and valleys might come into play as we emerge from COVID-19.

Using models built to study RSV and seasonal flu in the United States, she and her collaborators investigated how limiting transmission would change the pool of susceptible individuals. They expected that if people who would normally catch these viruses did not (since masks can stop many viruses, not just COVID-19), then the population susceptible to infection should grow over time as immunity wanes.

Even so, what they found was surprising. According to the models, even six months of a 20% reduction in transmission — a conservative estimate — would result in a spike of RSV cases after the restriction ends. That was true even when the restriction happened during the summer, fall, or spring, when RSV typically spreads slowly, if at all.

The models also suggest that flu cases may spike after people stop distancing and wearing masks. The group expects similar patterns may exist for other respiratory viruses that also provoke only temporary immunity. “Public health officials should know that they could see this,” Baker says. 

The increase in susceptible individuals is far from the only factor in the timing and size of outbreaks, cautions Katia Koelle, a biology professor at Emory University who studies viral dynamics. Flu, in particular, engages in a constant bait-and-switch with the immune system, evolving new forms that our defenses don’t recognize. Those shifts are hard to predict, and they typically determine the size of flu seasons more than immunity.

Still, it makes sense to expect that a period of reduced transmission may put us at risk of bigger outbreaks, she says. “Qualitatively, we should expect [these viruses] to be circulating at higher levels.”

So when might we expect this potential post-COVID viral onslaught?

We may not necessarily see it this year,” Baker says. Her models suggest that if people begin to gather again in the spring or summer, after the COVID-19 vaccine becomes widely available, this projected big wave might not arrive until the end of 2021.

It may sound daunting to consider a worse-than-usual winter virus season, especially after the COVID surge of this winter. However, there are ways we can prepare ourselves, scientists argue. 

Vaccination will be important — for COVID-19 and everything else. RSV doesn’t have a vaccine, but the flu does. “It will be even more important for people to get the flu shot next year,” says Koelle, especially for children, who tend to be serious spreaders.

Vaccination rates of children have declined during the pandemic; in July, the World Health Organization and UNICEF warned that this trend is laying the groundwork for future outbreaks of preventable diseases like measles. In Michigan, for instance, vaccines other than flu shots administered to children were down 21.5% in the early months of the pandemic compared to previous years, researchers report; in May 2020, they note, fewer than half of 5-month-olds were up to date on their shots. Parents and doctors often canceled well-child visits to avoid COVID exposure, but all too often, the appointments were not rescheduled.

Recovering that lost ground will be crucial in making the period after the pandemic smoother. In New York City, public health officials made a push this past summer to urge pediatricians to get their patients caught up on their shots; numbers did climb, officials reported in July 2020, though not to pre-pandemic levels.

Over the longer term, public health officials could monitor immunity levels through routine surveillance of blood samples collected across the country, checking for antibodies against key viruses, says Michael Mina, a professor of epidemiology at the Harvard School of Public Health and a physician at Brigham and Women’s Hospital in Boston. Tests could show when collective immunity against a virus is waning and the risk of a big outbreak is increasing.

With information about how many people in the population lack immunity to a given virus, public health officials could act before a big outbreak occurs, boosting vaccination and other preventative measures.   

Eventually, members of the public could use the information from this observation network to make decisions, Mina suggests. Instead of vaguely knowing that something’s going around at school, parents could potentially check a public database via an app to know whether the scourge of their community is a low-risk rhinovirus, for instance, or RSV, which is more serious. Mina’s team is working on a smaller-scale demonstration of this idea, which it calls the Global Immunological Observatory, using plasma samples from the last year nationwide to look at COVID-19.

It’s also possible that our experience with the pandemic has fundamentally changed our behavior for the better. Perhaps pandemic-era know-how in contract tracing and managing outbreaks will not immediately be forgotten. Perhaps when RSV shows up again in schools and day-care centers, providers will be able to quell the spread before it gets out of hand. Perhaps cultural changes from the pandemic will persist, the way wearing a mask when ill became more common in Asia after SARS, or the way disposable paper cups replaced shared cups at public water sources after the 1918 flu.

“My hope is that we take disease transmission risk more seriously in settings like airplanes, schools, workplaces, [and] long-term care facilities,” Scarpino says. Those settings “need to be prepared to quickly determine whether an outbreak of respiratory illness is COVID-19 or a less deadly pathogen, and respond accordingly.”

An awareness that the pandemic is affecting other viruses will go a long way, researchers say. Perhaps we can use these experiences to shine a light ahead, instead of proceeding onward in the dark.

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Veronique Greenwood's work has appeared in the New York Times Magazine, The Atlantic, National Geographic, and many others.


Illustration by Jack Dylan


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