The maps that the National Weather Service publishes every month, forecasting what may happen in the months to come, predict above-average temperatures and rainfall for much of the United States in May, June, and July. In normal times, that would be unnerving news, a sign of storms coming and a climate wrenched awry. Right now, though, the predictions are fueling a weird hope: that a hotter, wetter summer might put the brakes on Covid-19.
In the temperate zones of the world, other respiratory pathogens, and even other coronaviruses, lose their power as temperatures and humidity rise. But for the coronavirus fueling this pandemic, that remains only a hope. The research showing whether it possesses what virologists call “seasonality” is early, and much of it is contradictory. There is no clear proof yet that summer might save us.
The notion that the virus might be seasonal got big air time on April 23 during a White House briefing, when William Bryan, an acting undersecretary in the Department of Homeland Security, previewed unpublished research done at the National Biodefense Analysis and Countermeasures Center, within the Army’s biosecurity laboratory. Bryan said that increases in ambient temperature and humidity, and exposure to sunlight, all cut into the virus’s ability to survive in the droplets that people exhale when they cough and speak, as well as on the surfaces those droplets fall onto.
Higher temperatures, higher humidity, sunlight: That sounds like summer. But the experiments that showed the suppressive effects were done in controlled laboratory conditions. Though vice president Mike Pence said during the briefing, “The fact that heat and sunlight have on the coronavirus … increase the confidence that we feel about the coming summer,” Bryan was careful to hedge. “It would be irresponsible for us to say that we feel that the summer is just going to totally kill the virus,” Bryan said.
Two weeks before that briefing, a committee at the National Academy of Sciences published what’s called a “rapid expert consultation”—a review of published research and preprints, combined with interviews—about whether the virus might be affected by changes in temperature, humidity, or time of year. The committee members expressed a more nuanced view than the White House did. “Although experimental studies show a relationship between higher temperatures and humidity levels, and reduced survival of SARS-CoV-2 in the laboratory,” the group wrote, “there are many other factors besides environmental temperature, humidity, and survival of the virus outside of the host, that influence and determine transmission rates among humans in the ‘real world.’”
David A. Relman, a physician and professor at Stanford University School of Medicine, wrote the opinion for the group, which reviewed extant and not-yet-published research, including the Army work. Speaking by phone, he said that though there is some experimental evidence that the virus is sensitive to heat and humidity, and some observational natural history studies of transmission rates varying in places with different ambient temperatures and humidity, there is not enough solid evidence to confirm that a summertime improvement might occur. “What we’re saying is: It may happen and it may not,” Relman said. “Just don’t count on it.”
The reason everyone’s so interested in whether summer can kill the novel coronavirus is simple: So far, not much else has worked. The interventions that have had some effect—such as social distancing and broad shutdowns of businesses and schools—impose enormous financial and psychological costs. Trusting in summer would be an uncomplicated solution. And there’s logic behind that hope, because other respiratory infections, such as the flu and some colds, are largely seasonal.
But why they are is a bit of a mystery. Researchers have speculated that changes in ambient temperature and humidity may make these viruses less viable by affecting their outer proteins and membrane, or by altering how quickly the virus-carrying droplets from mouth and throat evaporate. On the other hand, the change in infection rate might be due to the ways that people change their behavior with the seasons, going from being enclosed in offices and schools to opening windows and spending more time outdoors. (Getting more sunlight, which drives Vitamin D production and might enhance the immune system’s defenses, might play a role, too.)