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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 probably 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.

As nail salons, restaurants, and gyms began to reopen on May 15 in parts of Virginia, business owners and customers are proceeding with caution. (The Washington Post)

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.

President Trump on April 23 suggested doctors should look at using light and heat on the body to see if it helps covid-19 patients. (The Washington Post)

“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. Covid-19 is spreading fiercely around the world, in all kinds of weather conditions,” Tomas Molina, the chief meteorologist at Spain’s Televisió de Catalunya and 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, Massachusetts Institute of Technology 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.

Even in summer, most people live their lives indoors, and much of what happens this summer will pivot on how carefully people maintain social distancing and limit contact with other people. In communities that ease the shutdown restrictions, some people will return to office buildings and residences. Viral transmission has been common in confined spaces where people are in close contact.

Another new study, from researchers at Princeton University and the National Institutes of Health, found that our lack of immunity to the coronavirus will overwhelm any tempering influence that warm, humid weather may have on the virus’s spread. Only in future years, if the virus transitions to an endemic illness that flares up in smaller outbreaks each year, will climate be a more important factor, the study found.

Research published in recent days, looking at how human speech creates small respiratory droplets that can linger in the air for many minutes, has raised anew the question of how the virus spreads and whether some transmission is through these small aerosol droplets. That remains unresolved.

David Rubin, director of PolicyLab at Children’s Hospital of Philadelphia, and his colleagues have incorporated weather factors in the model they have developed showing when and where it will be relatively safe to ease some shutdown orders.

“Clearly, I believe weather is impacting it — it’s just not impacting it enough to completely eliminate transmission,” Rubin said. “That’s why we’re still seeing cases in Florida and Texas and Tennessee. It seems to be preventing a big exponential rise in cases.”

Multiple early studies provide evidence of statistical ties between temperature and humidity ranges and the geographic regions where this virus has thrived. While none of these studies has been conclusive, they all point to the same general possibility: The pandemic could ease in parts of North America and Europe during the summer months, although it could come roaring back in the fall.

Rich Sorkin, co-founder of Jupiter Intelligence, a risk management company that is helping clients understand the effect of weather on the coronavirus, said, “There’s a certain element of geography-is-destiny here.” The countries with the largest outbreaks and highest mortality rates to date are all in cooler climates, he said.

“There’s a strong pattern of weather characteristics influencing mortality,” he said. But he added that government policies and other aspects of the virus are also important.

The Trump administration has touted laboratory studies, carried out at the U.S. Army’s high-level biosecurity laboratory at Fort Detrick, Md., as revealing the virus’s susceptibility to heat and sunlight. The results, revealed during an April 23 coronavirus task force news briefing, largely matched other laboratory studies and the suspicions of some researchers by showing that the novel coronavirus, like many other viruses, does not survive as long on certain surfaces and in the air when exposed to high amounts of ultraviolet light and warm and humid conditions.

But David Heymann, a professor at the London School of Hygiene & Tropical Medicine, said laboratory studies on the coronavirus’s behavior under different weather conditions should be viewed with caution.

“Laboratory studies are just that, and they’re not the real situation,” he said. “We still see it transmitting in most parts of the world, even in tropical areas.”

Epidemiologists, Heymann said, are looking at what is happening in real settings, such as the clusters of cases in meatpacking plants and nursing homes, both of which are confined spaces with people in close contact. Laboratory studies, he said, should follow such observations to test how best to protect people in those settings, rather than having lab results lead directly to policies that may not reflect where and how people are getting sick in the real world.

“That’s always been a disconnect between laboratories and epidemiologists,” he said.