Warm weather probably won’t slow COVID-19 transmission much

Any seasonal benefit can be canceled by humanity’s vulnerability to the virus, a study suggests

woman wearing mask in sunlight

While some viral diseases wane with the arrival of summer, experts caution that so far, evidence suggests that won’t be the case for COVID-19.

Rytis Bernotas / iStock / Getty Images Plus

The arrival of spring in the Northern Hemisphere has raised hopes that warmer and wetter weather might slow or even stop the COVID-19 pandemic, at least until fall. But don’t plan on that happening, U.S. health experts say.

“One should not assume that we are going to be rescued by a change in the weather. You must assume that the virus will continue to do its thing,” Anthony Fauci, director of the U.S. National Institute of Allergy and Infectious Diseases in Bethesda, Md., and a member of the White House coronavirus task force, said during an interview April 9 on ABC’s Good Morning America.

A report released April 7 by the National Academies of Sciences, Engineering and Medicine also says that, while much about the virus remains unknown, summer temperatures probably won’t do much to dampen the spread of the virus. 

While scientists still don’t know if touching shared surfaces is a major driver of the pandemic, compared with direct person-to-person transmission (SN: 3/4/20; SN: 4/2/20), understanding how the virus fares in different environmental conditions could provide clues as to the likelihood of a summertime slowdown. Many viruses wither under high temperatures and there is some evidence that the same might be true for SARS-CoV-2, the novel coronavirus that causes COVID-19.

In an experiment using SARS-CoV-2 in a lab solution, increasing temperature decreased the amount of viable virus that could be detected, according to an April 2 study in the Lancet Microbe. No infectious virus remained after 30 minutes at 56° Celsius (133° Fahrenheit). And just five minutes at 70° C was enough to inactivate the pathogen.

But these temperature highs are rare, if not impossible, in the lower atmosphere. The National Academies’ report — aimed at updating the White House on how changing seasons might affect the pandemic — instead points to other, ongoing studies at national laboratories that could soon inform how the virus fares under a wider range of conditions.

Perhaps more relevant are studies looking for correlations between COVID-19 cases and local weather. If warmer, wetter places tend to have smaller outbreaks, then much of the Northern Hemisphere could be in for a break.

One early study of the outbreak, posted March 30 at medRxiv.org, suggested that for every 1 degree C increase in atmospheric temperature at relatively high levels of humidity, daily confirmed cases decreased by 36 to 57 percent in China’s Hubei Province. That pattern did not hold across mainland China, though.

Another study, released March 19 and later updated on the preprint repository SSRN, found that 90 percent of global transmission through March 22 occurred when temperatures were between 3° and 17° C. However that study, by a computational neuroscientist and environmental engineer at MIT, did not account for variables such as countries’ testing capacities or policy responses, says Maciej Boni, a Penn State epidemiologist. As a result, Boni doesn’t put much stock in the study’s conclusions.

“An epidemic is a dynamic process,” so research into a virus’s transmission ability needs to consider the many possible factors that might influence the results, Boni says.

The National Academies’ report notes that “studies published so far have conflicting results regarding potential seasonal effects, and are hampered by poor data quality, confounding factors and insufficient time since the beginning of the pandemic from which to draw conclusions.”

Because humanity has never before encountered this new coronavirus, the vast majority of the population is highly susceptible to infection (SN: 3/24/20). That widespread vulnerability will likely overwhelm any temperature effect on transmission rates, according to a study that modeled the effect of varying levels of seasonality on transmission, posted at medRxiv.org April 7.

That conclusion matches what countries like Australia and Brazil have experienced, with large outbreaks during their summer in the Southern Hemisphere.

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