Summer weather could help fight coronavirus spread but won't halt the pandemic, research shows
New research has bolstered the hypothesis that summer's heat, humidity, abundant sunshine and opportunities for people to get outside should combine to inhibit - though certainly not halt - the spread of the coronavirus.
But infectious disease experts add a cautionary note: Any benefit from summer conditions would likely be lost if people mistakenly believe the virus can't spread in warm weather and abandon efforts that limit infections, such as social distancing.
"The best way to think about weather is as a secondary factor here," said Mohammad Jalali, an assistant professor at Harvard Medical School who has researched how weather affects the spread of viruses.
The effect of weather on the coronavirus has been the subject of extensive research in recent months and is acutely relevant as the Northern Hemisphere edges closer to Memorial Day and the unofficial start of summer. States and cities are terminating or modifying shutdown orders, and millions of students trying to take classes remotely will soon see their disrupted school year come to an end.
In this transitional moment, many people who have been in quarantine will probably find themselves in places - beaches, pools, parks, recreational sites - that historically have been viewed as benign but now carry some hard-to-calculate risk of viral transmission.
Swimming in a chlorinated pool should be safe if people maintain the six-foot social distancing rule, according to new guidelines from the Centers for Disease Control and Prevention. The CDC encouraged the use of facial coverings but cautioned they should not be worn in the water, because when wet they can make it difficult to breathe.
"There is no evidence that the virus that causes COVID-19 can be spread to people through the water in pools, hot tubs, spas, or water play areas. Proper operation and maintenance (including disinfection with chlorine and bromine) of these facilities should inactivate the virus in the water," CDC spokeswoman Kate Grusich said in an email.
But people can still transmit the virus through close personal interactions in any conditions, inside or outside, in sun or rain. The global picture reveals that the coronavirus is capable of spreading in any climate. Warm-weather countries, including Singapore, Indonesia, Brazil and Ecuador, are enduring significant viral spread.
"Environmental conditions are just one more element of the equation, and not by far the most relevant. Covid19 is spreading fiercely around the world, in all kinds of weather conditions," Tomas Molina, the chief meteorologist at Spain's Televisió de Catalunya, who is also a professor at the University of Barcelona, said in an email. Molina examined the course of the outbreak in Barcelona and found a relationship between higher temperatures and lower virus transmission rates.
In recent weeks, numerous research studies, based on laboratory experiments, computer models and sophisticated statistical analyses, have supported the view that the coronavirus will be inhibited by summer weather.
A new working paper and database put together by researchers at Harvard Medical School, MIT and other institutions examines a host of weather conditions, from temperature and relative humidity to precipitation, at 3,739 locations worldwide to try to determine the "relative covid-19 risk due to weather." They found that average temperatures above 77 degrees are associated with a reduction in the virus's transmission.
Each additional 1.8-degree temperature increase above that level was associated with an additional 3.1 percent reduction in the virus's reproduction number, called R0, and pronounced "R naught." That is the average number of new infections generated by each infected person. When the R0 drops below 1, an epidemic begins to wane, although it doesn't happen overnight.
However, like previous studies, the research from Harvard and MIT found that the transition to summer weather won't be sufficient to completely contain the virus's transmission.
Other coronaviruses, such as SARS and MERS, have exhibited seasonality, ebbing during periods of warmer weather much like the seasonal flu. Many experts have suspected for months that the novel coronavirus might do the same.
The seasonal factors in virus transmission work the other way around, too: A decline in transmission in summer would probably be followed by a seasonal increase in infections in the fall.
There are many factors in the seasonal pattern. The virus degrades outside a host cell, and does so more rapidly when exposed to heat or ultraviolet radiation from the sun.
Humidity plays a complex role. Research indicates that viruses easily spread in winter in the dry air of climate-controlled spaces. By contrast, higher humidity makes respiratory droplets, the most common vector of virus, drop to the ground or floor more quickly, limiting airborne transmission.
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