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

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, 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. 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.