Flu Comes Back Every Year. Will Coronavirus?

Could spring be the thing that saves us from the novel coronavirus? President Trump has suggested as much, telling a rally that " heat generally speaking kills this kind of virus" so "[the virus] will go away in April." It's not a totally bizarro suggestion, given that flu and polio and a lot of other viruses really are seasonal, usually with transmission spiking in the colder winter months.

But - and you knew the "but" was coming - experts say we don't yet know enough about coronavirus to tell whether it's likely to be seasonal. More importantly, though, a virus doesn't just "go away" because it's warm. If coronavirus cases drop off in summer, that just means there's a risk of it resurging in fall. And if it does come back - that means our efforts to actually stop it have failed.

The factors that make some viruses seasonal are complicated and, in some ways, still kind of a mystery, said Wan Yang, professor of epidemiology at Columbia University. Heat does play a role, she said. Viruses tend to survive better in cold - kinda the opposite of how bacteria do better in warmth. "That's why we have more food poisoning in summer," she said.

But humidity turns out to be a bigger player here. It comes down to physics, much of which researchers are still trying to understand, said Spencer Fox, a data scientist who has published research on the seasonality of the flu and the epidemiology of viruses. A virus like the flu spreads when people sneeze and spray aerosolized droplets of gunk and virus into the air. Humidity affects how long those droplets hang out where other humans can breathe them in and how well the virus can survive inside the droplet.

One theory is that the low humidity in winter means the liquid coating protecting the virus droplet doesn't evaporate as quickly, Yang said. This theory would also explain why tropical areas have a flu season associated not with cold weather but with extremely high humidity - another situation when there's not much evaporation happening. In between those extremes, the liquid in the virus-carrying droplets tends to evaporate, concentrating solids and making an environment less hospitable to viral survival, she said.

Again, though, this is from research on the flu. It doesn't necessarily mean the virus that causes COVID-19 would work the same way. The thing we call a "common cold" can sometimes be caused by a coronavirus, and Yang said colds aren't as strongly seasonal as the flu is. Another counterexample, Yang said, is SARS, which in 2003 made some people sick in late spring in Toronto - in other words, a place with warmer conditions similar to those in much of the United States. (SARS was also caused by a coronavirus - there are many.) In other words, waiting for the season to change should not be your plan for stopping the spread of a novel virus.

Which brings us to the other part of the problem. "SARS didn't just disappear," Yang said. Unlike the flu, SARS flared up on the global stage and then vanished. It hasn't come back seasonally. But there's nothing about the SARS virus itself that made it a one-time thing, somehow physically different from perennial viruses that pester us on an annual basis. "It disappeared because of very stringent public health control and intervention," she told me. "If there weren't such control it might still be circulating."

Turns out, another of the biggest factors in whether a virus becomes seasonally recurrent is whether it reaches a level of transmission that is pandemic (prevalent everywhere) and endemic (circulating constantly in local human populations). You can stop that from happening, as with SARS, by tightly monitoring the spread of a novel virus - watching the people who have been exposed, isolating the people who are sick, cutting off the virus's access to new hosts.

But if you don't do that - or you don't do it effectively - a new virus can easily become a recurrent one. Flu is pandemic and endemic. It doesn't disappear every summer. It's just biding its time, hanging out in human bodies (either at low levels of infection or in the opposite hemisphere), ready to spread again once conditions improve. SARS never became pandemic and endemic - largely because of a lot of human effort, and partly because of a convenient tendency to not be very transmissible until victims showed symptoms of infection.

But the new COVID-19 virus doesn't seem to offer us that handicap. There seem to be asymptomatic cases, and mild ones that people don't recognize as dangerous. And there's evidence suggesting people can spread the virus before they're even showing symptoms themselves. That makes it more challenging for public health experts to stop, Yang said. When I asked her how likely it was that COVID-19 could become seasonal, she laughed ruefully. "Nobody knows," she said. "The WHO still says there's hope we can get this under control."