Critical to a virus' resilience outside a body is the integrity of its lipid sheath, a fatty substance with a gel-like consistency in cooler air, according to National Institutes of Health research on flu viruses. Like butter in a hot pan, that layer becomes more liquid and less protective of the virus in warmer weather. Drier air may also help a virus' spread, a 2007 study on the flu from Mount Sinai School of Medicine in New York City found, possibly because lower humidity dries out the nose's protective mucus and allows viral particles to stay aloft longer, since they are less likely to be weighed down by water particles in a dry environment.
Those findings, though, have exceptions depending on the virus, and aren't universally accepted. There also isn't enough research yet to clarify whether they apply to COVID-19.
An April study published in the medical journal The Lancet found the virus didn't survive as well in higher temperatures. In September, researchers from the University of Nicosia in Cyprus looked at the effects of temperature, humidity, and wind on COVID-19 and found saliva droplets contaminated with the virus evaporated faster in high temperatures. The study also found low humidity caused droplets to evaporate more quickly.
If climate has any effect on the virus' spread it will probably be most pronounced in a few months, Kessler said. Flu season tends to be from August to October, but seasonal coronaviruses similar to COVID-19 hit hardest in colder months.
"Seasonal coronavirus transmission peaks pretty reliably in the northern hemisphere some time in December or January," Kessler said. "We're a little bit more likely to see these resurgences of infection a little bit later in the year."
Most recently, a group of researchers from Boston Children's Hospital published a study in Nature this month that evaluated the virus' spread in China. Researchers concluded the effects of temperature and humidity on the pandemic were negligible, at best.
"Sustained transmission and rapid growth of cases were observed over a range of temperatures and humidity conditions ranging from cold and dry provinces in China, such as Jilin and Heilongjiang, to tropical locations, such as Guangxi and Taiwan," the study found.
What seemed to matter most in slowing transmission rates, the study found, were nonpharmaceutical interventions. In other words, keep wearing a mask and limiting indoor gatherings.
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