Does Warm Weather Curb Coronavirus? What the Experts Say

(Bloomberg) --

In temperate climates, seasonal flu tends to flare in winter and recede as spring arrives. That gave rise to hopes by some that the arrival of warmer weather in the northern hemisphere would keep Covid-19, also transmitted by droplets expelled from the nose and mouth, in check there. The evidence so far suggests the change of seasons has an effect on transmission of the coronavirus -- but a modest one that can’t be relied on to stanch the pandemic.

1. What’s the evidence?

Researchers who examined the spread of Covid-19 and local weather conditions in more than 3,700 global locations through late April concluded in a working paper that temperatures higher than 25 degrees Celsius (77 degrees Fahrenheit) were associated with lower transmission rates. Their study, posted in May, has not yet been published in a scholarly journal, which requires papers to be vetted by experts working in the same field. Earlier analyses based on smaller data sets had similarly associated higher rates of transmission with mild temperatures as well as mild humidity.

2. What’s the expected effect?

The authors of the May working paper projected that summer may provide “partial relief” to some regions but without quelling Covid-19 outbreaks. Regions experiencing reduced risks in summer, they said, would face increased risks in the fall. In a paper published the same month in Science, another group of researchers concluded that seasonal effects would be insignificant during the pandemic stage of the coronavirus, when the main driver is the large number of people who have never been exposed to the virus before and have no immunity. Weather may begin to play a bigger role, they said, if the virus becomes endemic, or well-established.

3. How does weather change things?

It’s not clear, but flu’s behavior offers clues. In temperate regions, flu is largely a winter phenomenon, whereas in tropical and subtropical areas it tends to occur during the rainy season, if there is one, or year-round if not. There are various theories why. There’s evidence that both cold, dry air and especially humid conditions are favorable for flu’s transmission. That may have something to do with human behavior: Just as people tend to crowd indoors in cold and rainy weather, which could boost contagion, they spend more time outside in fine weather, reducing contamination. There’s the possibility that summer lifts people’s melatonin and vitamin D levels, which can enhance their immune systems, and that viruses lose their punch in warmer weather because their fatty coating degrades.

4. Where does flu go in summer?

The easing of flu outbreaks in the summer doesn’t mean the virus dies out. Some evidence suggests it migrates to the opposite hemisphere, before returning again with cooler weather.

The Reference Shelf

• Related QuickTakes on understanding the pandemic, lessons from the response, the race for vaccines and drugs, and the guidance on masks.
• The director of the U.S. National Institutes of Health blogs on the seasonality question.
• The article in Science, the working paper published in May and a related data visualization.

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