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No, warm weather will not kill the coronavirus

Experts and studies suggest no one should get their hopes up as temperatures rise and spring turns into summer.

A mask-wearing man set up a hammock between two trees along the Charles River on the Esplanade.
A mask-wearing man set up a hammock between two trees along the Charles River on the Esplanade.Jim Davis/Globe Staff/file

For months, President Trump has claimed that the novel coronavirus will likely vanish with warmer weather. Massachusetts health officials have leaned on Mother Nature, too, suggesting the state’s high infection rate is linked to our cold winter climate.

The latest came Thursday at Trump’s daily press briefing when William Bryan of the Department of Homeland Security presented “emerging” evidence from a government lab suggesting that sunlight, heat, and humidity weaken the coronavirus on surfaces and in the air.

But disease experts and recent studies suggest no one should get their hopes up as temperatures rise and spring turns to summer. Mounting evidence indicates that COVID-19 is unlike the seasonal flu, which does dissipate as the weather warms. Most studies have found weak evidence that high temperatures and humidity will vanquish the virus.

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“We all hope that if we can’t do something about it before June, that it all disappears on its own,” said David Walt, a pathology professor at Harvard Medical School and Brigham and Women’s Hospital. “But all indications from other parts of the world suggest otherwise.”

Though much about the virus remains unknown, experts across the globe are racing to understand the roots of the pandemic and what factors might help quell outbreaks.

A recent report from the National Academies of Sciences, Engineering and Medicine downplayed the impact of warm weather, concluding it “may not lead to a significant reduction” in spread, at least on its own. The report, coauthored by Walt and issued to the White House earlier this month, found that the best defenses are the adoption of major public health actions, such as social distancing, and more widespread immunity to the virus.

The report acknowledged the limited and, at times, conflicting evidence, based on a review of studies from around the world. But it didn’t give much weight to claims that COVID-19 spreads less efficiently in hotter places.

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“Given that countries currently in ‘summer’ climates, such as Australia and Iran, are experiencing rapid virus spread, a decrease in cases with increases in humidity and temperature elsewhere should not be assumed,” it said.

So while warming temperatures can dampen the flu and other respiratory infections, their impact on the coronavirus remains a bit of a mystery, according to Cindy Prins, a University of Florida associate professor of epidemiology.

“We still don’t know whether there is any effect on COVID-19 with warmer and more humid weather,” Prins said.

“If it sticks around and keeps coming back as a seasonal virus, then we’ll probably start to understand that a little bit better," Prins said. "But I don’t necessarily expect to see an effect of that at least this year.”

But as the virus continues its spread and the death toll grows, the hope of a warm-weather link to fewer infections is hard to shake. Especially for government officials.

Asked last week why the infection rate in Massachusetts was so much higher than that of Washington state, which has roughly the same population, the Massachusetts health department pointed in part to New England’s colder climate.

“Though the science is not yet conclusive, many published articles indicate that COVID spreads more easily in colder weather (similar to most strains of influenza),” the department said in a statement to the Globe.

The statement went on to say, “The climate in Massachusetts in February and March is notably colder than Washington, which likely contributed to greater spread.”

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The department did not cite the articles on which it based its assumption.

Asked about that thinking again Thursday, the department said its comments still stand, but noted that state health officials are “focused on communicating" that citizens should stay home, wear face coverings, and socially distance from others.

Even if weather was a factor, the department’s argument was ill-timed. Typically Boston winters are significantly colder than Seattle’s, but not this year. Boston had an unseasonably warm winter with an average temperature of 37.8 degrees in December, January, and February.

“This winter season of 2020 was the second highest in recorded history,” which goes back to 1877, said Bill Simpson of the National Weather Service’s Boston office.

With an average temperature nearly 6 degrees above normal, Boston was not much colder than Seattle, where temperatures averaged 41.8 degrees, according to National Weather Service data. And that trend continued in March.

Dr. Peter Hotez, dean of the National School of Tropical Medicine at Baylor College of Medicine in Houston, said such state-to-state comparisons of weather and coronavirus infections are shaky.

“There no evidence for that, from what I know,” he said.

Still the high heat and humidity theory persists.

At Trump’s press briefing on Thursday, Bryan of the Department of Homeland Security said new science shows that the coronavirus may be vulnerable to both heat and light.

“Our most striking observation to date is the powerful effect that solar light appears to have on killing the virus,” Bryan told reporters. "Summerlike conditions are going to create an environment where the transmission can be decreased and that’s an opportunity for us to get ahead.”

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A reporter then asked why the virus is still growing in some hot and humid places in the US, such as New Orleans.

Undeterred, Bryan said the department’s new information could help governors make decisions about how and when to reopen their states.

“This [research] is just another tool in the tool box, another weapon in the fight that we can add,” he said.

As scientists continue to study what factors may influence transmission of the coronavirus, Walt, the pathology professor from Harvard Medical School, said health officials need to keep in mind one very basic human trait.

“Even if [the coronavirus] were very sensitive to heat, we would have to tell people on 95-degree days, don’t turn your air conditioning on" to get the virus-killing benefit, Walt said. That, he added, could be a hard sell.


Kay Lazar can be reached at kay.lazar@globe.com Follow her on Twitter @GlobeKayLazar. Andrew Ryan can be reached at andrew.ryan@globe.com Follow him on Twitter @globeandrewryan.