Will Warm Weather Stop the Coronavirus?

In temperate climates, the seasonal flu tends to flare in winter and recede as spring arrives. But what about the new coronavirus? Will the arrival of warmer weather keep it in check? U.S. President Donald Trump has weighed in, saying in a televised address that April sunshine could chase away the scourge. Health experts say it’s more complicated than that. Just because outbreaks of influenza wilt with seasonal changes doesn’t mean a different virus will behave in the same way.

1. Is the climate a factor?

It’s too early to know. The new coronavirus, which causes a respiratory disease called Covid-19, has infected more than 120,000 people worldwide, but it only appeared in central China at the end of last year. Officials of the World Health Organization said March 5 there’s no reason to believe temperature will play a role in the outbreak but that the subject was worth investigating.

2. Is there evidence either way?

A few researchers have prepared analyzes on the subject, but none have been yet been published in scholarly journals, which require papers to be vetted by experts working in the same field. A group of U.S. and Iranian researchers concluded that the places Covid-19 infection has mostly taken hold so far -- such as the Chinese city Wuhan, Milan and Seattle -- share similarly mild humidity and temperatures ranging from about 5 to 11 degrees Celsius (41 to 52 degrees Fahrenheit). That suggests the virus may fade as weather warms in those areas. The authors, however, said predictions should be regarded with “extreme caution.” And there are exceptions to their observation. For example, the virus has made inroads in Singapore, which has a warm and humid climate.

3. Why does flu vary with the seasons?

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. Just as people tend to crowd indoors in cold and rainy weather, boosting 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 boost their immune systems, and that viruses lose their punch in warmer weather because their fatty coating degrades.

4. What about SARS?

The 2002-2003 epidemic of severe acute respiratory syndrome did indeed ease over the summer. But that may have had little to do with the weather. “SARS did not die of natural causes,” Harvard epidemiologist Marc Lipsitch writes. “It was killed by extremely intense public health interventions in mainland Chinese cities, Hong Kong, Vietnam, Thailand, Canada and elsewhere.” Among other things, authorities isolated cases and quarantined their contacts. The strategy worked, Lipsitch says, because the most infectious SARS patients showed obvious symptoms. While China has taken similar steps with the new coronavirus, its long incubation period and many mild cases have complicated containment efforts.

5. Where does flu go in winter?

The easing of flu outbreaks doesn’t mean the virus dies out in warmer temperatures. It just don’t transmit as easily. So, even if the coronavirus does subside in the summer, it could return in the fall.

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