Does Hot Weather Kill Coronavirus?

Isn't summer supposed to slow down the new coronavirus? The hot weather might help, say Covid-19 experts, but here's why it's hard to know for sure.

Hot weather was going to give us relief from Covid-19. But instead of slowing down this summer, the novel coronavirus is picking up speed with the United States now seeing over four million positive cases in the rear-view window, according to Johns Hopkins University. Initially, five months ago, we had pinned our hopes on the examples set by other viruses, like the flu and some common colds, which tend to wane in warmer weather. But temperatures, if they affect the virus at all, can be overshadowed by many other factors.

“There are probably too many things that we just don’t know, so many variables,” says Aaron E. Glatt, MD, a spokesperson for the Infectious Diseases Society of America (IDSA). “It’s like saying hot weather will help melt the snow but not if you’re in a refrigerator.”

woman shielding eyes from large coronavirus cellsKlaus Vedfelt/Getty Images

Virus survival and hot weather

Climate does seem to play a role in the seasonality of some (though not all) viruses. Think of influenza and the common cold, which tend to start in the autumn. Various environmental factors may explain some of this. Low humidity levels in winter can speed up flu transmission as well as the spread of respiratory syncytial virus (RSV). Middle Eastern Respiratory Syndrome (MERS), a virus similar to SARS-CoV-2 (the virus that causes Covid-19) which first appeared in Saudia Arabia in 2012, also waned in warmer weather. Some experts have speculated that because levels of vitamin D drop in winter (the body needs sunshine to make the vitamin), this could undermine the immune system and leave people more vulnerable to infections. (Can UV light kill coronavirus?)

Indoors vs outdoors

But environmental factors may have less to do with winter transmission than the fact that people are more likely to huddle indoors. “The optimal temperature for viruses to replicate oftentimes is the temperature of warm-blooded animals which is pretty hot, close to 100 degrees Fahrenheit,” says Iahn Gonsenhauser, MD, chief quality and patient safety officer at The Ohio State University Wexner Medical Center in Columbus. “One of the things we think contributes to decreased virus numbers is decreased ease of transmission in warm months because we’re seeing fewer people with sniffles and coughing and sneezing stuck indoors.” Cold weather also causes some respiratory drainage, making it more likely germs will escape and travel.

Covid-19 and the change in seasons

Covid-19 is so new, scientists are still learning about the pathogen. But experts had reasons to suspect warm weather might cool off Covid-19, so to speak. One analysis of 50 U.S. cities, published on June 11 in JAMA Network Open, noted that Covid-19 outbreaks were located on similar latitudes with similar temperatures (41 to 52 degrees Fahrenheit) and lower humidity. This suggests that the virus might not fare as well in hotter zones. (Here are the signs that you already had Covid-19.)

Sunlight may also cause SARS-CoV-2 to die faster on surfaces, though no one knows what that means in terms of actual infections, according to The Ohio State University: The virus is way more efficient at spreading from person to person, not from inanimate surfaces. (This is how long coronavirus lives on surfaces.)

Covid-19 and immunity

Experts believe one of the main reasons Covid-19 isn’t going away is the fact that everyone lacks immunity. Because the virus is brand new, humans don’t have any existing protection against it. A study published on July 17 in the journal Science used two viruses very similar to SARS-CoV-2 to build estimates on how the novel coronavirus might spread and the factors that could slow it down. The researchers concluded that our lack of immunity was indeed driving the spread. For example, when the common cold or influenza rolls around, our bodies fought off versions of it before and can muster some defense against the viruses. When more people have had SARS-CoV-2, we may start seeing seasonal fluctuations, experts say. (This is what you need to know about Covid-19 antibody tests.)

Covid-19 and human behavior

While the Science study didn’t find any effects of warmer weather on SARS-CoV-2, it did identify measures that could slow the spread: Social-distancing, mask-wearing, and other common-sense protective behaviors. (Learn the difference between self-isolation and quarantine.) And that brings us to human behavior, another factor that seems to be pushing any effects of weather to the background. (Find out if loud talking spreads Covid-19.)

“Viruses don’t actually do anything other than evolve to exploit our weaknesses and our predispositions,” says Dr. Gonsenhauser, “Behaviors would do almost everything we need to mitigate the risk, which we’re not doing. Our biggest weakness right now is squabbling over masks and social distancing. That opens channels of weakness that the virus that can take advantage of. If 80 percent or more are wearing masks in public, this virus goes away.”

girl standing on question markKlaus Vedfelt/Getty Images

So, does heat kill Covid-19?

Right now, other factors—namely our lack of immunity and our behavior—seem to be overshadowing any role weather might play. “We’re obviously seeing significant numbers in warm countries and warm states like California, Florida, and Arizona,” says Dr. Gonsenhauser. “The warm weather doesn’t seem to have mitigated it.” That doesn’t mean weather might not play a role in the future, turning Covid-19 into the kind of seasonal virus we see with influenza.

If heat doesn’t help, what will?

Let’s go back to social distancing and other behavioral changes. “We should be avoiding close contact in crowded spaces. We should be social distancing wherever possible and keeping our circle of close contacts as small as we can,” says Dr. Gonsenhauser. “We should be wearing masks whenever we’re out in public and whenever we’re with anyone who is not part of our household.” We should also be washing our hands with soap and water as often as possible, or alcohol-based sanitizer if soap and water aren’t available.

Summer’s secret weapon

The balmy days of summer offer one advantage in the struggle against Covid-19: more time outdoors. “You’ll clearly transmit any virus more in a closed poorly ventilated tight space than in an outdoor setting,” says Dr. Glatt, who is also chairman of medicine and hospital epidemiologist at Mount Sinai South Nassau. These strategies, adds Dr. Gonsenhauser, are “simple and we’re all capable of doing them. The weather’s not going to save us but we still can.” (Learn more about how to prevent Covid-19.)

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Amanda Gardner
Amanda Gardner is a freelance health reporter whose stories have appeared in cnn.com, health.com, cnn.com, WebMD, HealthDay, Self Magazine, the New York Daily News, Teachers & Writers Magazine, the Foreign Service Journal, AmeriQuests (Vanderbilt University) and others. In 2009, she served as writer-in-residence at the University of Wisconsin School of Medicine and Public Health. She is also a community artist and recipient or partner in five National Endowment for the Arts grants.