Does warmer weather slow coronavirus?

California was hit with the coronavirus emergency before New York, New Jersey and Chicago. But still, the Golden State has fared — by every measure — far better than colder places like the Big Apple.

The total cases of COVID-19 in California stands at just one-fifth of that in New York State. And California has recorded 10% of the deaths.

It’s not just California. Texas has also seen a coronavirus problem nowhere near as widespread or as deadly as New York. Neither has Florida. All four states have enormous populations, they all have densely packed urban centers and they boast international hub airports.

Why then, researchers are asking, is the global pandemic that ravaged New York playing itself out so differently in the South and the West?

“For every increase in heat of 1 degree Celsius (the equivalent of 1.8 degrees Fahrenheit), we are seeing about 2% decline in transmission,” public-health expert Ali Mokdad, the chief strategy officer for population health at the University of Washington’s Institute for Health Metrics and Evaluation, told ABC News. “We find this relationship in our data and possibly it would be more when the weather warms up this month.”

It was early on in the pandemic when President Donald Trump mused out loud that the global contagion would not pose a threat to the US because, like the flu, it would dissipate come spring.

“There’s a theory that, in April, when it gets warm — historically, that has been able to kill the virus,” Trump said on Feb. 14. “So we don’t know yet; we’re not sure yet. But that’s around the corner.”

The virus certainly did not disappear but there was, in fact, scientific logic behind Trumps words. And the pathogen’s course has given experts much to think about.

While there is no consensus yet, researchers now believe that the outside temperature and, possibly humidity, plays a role in making the virus spread less aggressively or intensely — even where people may be living in close quarters without social distancing measures.

“We can’t say for certain, but differences we are seeing could be partly related to differences in weather,” said ABC News contributor Dr. John Brownstein, an infectious disease expert and professor at Harvard Medical School who monitors viral trends around the world.

Take the Houston and Miami metropolitan areas. They have population densities and demographics similar to much of the New York City boroughs of Queens, Brooklyn and the Bronx that were, literally, overwhelmed by coronavirus in late March and early April. But the infection and death rates are markedly lower, with Miami-Dade County’s roughly 14,000 confirmed cases and 500 deaths compared to New York’s nearly 179,000 confirmed cases and over 14,700 deaths.

Both New York and Texas reported their first COVID-19 cases within days of each other — March 1 and March 9, respectively — but the disparity between their numbers of confirmed cases is staggering: 335,395 for New York; 38,869 for Texas, as of Sunday.

“Climate is an important driver for the seasonality of infectious diseases,” Brownstein said, and coronavirus may well follow the pattern.

It’s no coincidence to health investigators, Brownstein said, that coronavirus, once thrust unwittingly into the New York winter by travelers from Europe became such a viral inferno.

“The combination of cold-weather climate, high population density and increased use of public transport likely created a perfect environment for a novel respiratory virus to move efficiently through the population,” Brownstein said.

Researchers now want to figure out what it could mean if coronavirus follows the lead of the influenza of peaking in the cold and wilting in the warm.

Doctors believe increased dryness and close contact indoors could be two factors that promote transmission of infections during the wintertime cold. Warmer climates, on the other hand, offer many possible factors that could explain why disease transmission could be reduced, Brownstein said. Those are: greater sunlight, more humidity and the natural human urge to spend more time outside breathing in air that’s been filtered by Mother Nature as opposed to a building’s ductwork.

A study from engineers at the Massachusetts Institute of Technology, still awaiting peer review, offered a direct comparison between the spread of coronavirus and local environmental conditions. It determined that places with high growth rates like Italy, New York, and Washington state exhibited “weather patterns similar to original hotspots of Hubei and Hunan (China),” where the pandemic started. Those locations were averaging temperatures between 37 and 50 degrees Fahrenheit at the time. By contrast, places with warmer climates like Saudi Arabia, Australia, Qatar, and Taiwan have exhibited lower growth rates.

The researchers used weather data over 10-day periods between Jan. 22 and March 21 and concluded that the lower number of COVID-19 cases in tropical countries might be due to “warm-humid conditions, under which the spread of the virus might be slower as has been observed for other viruses.”

The study’s authors, in an email to ABC News, wrote their “main findings pointed towards the role of humidity as most important.” They did, however, stress that “there were found COVID cases even in the most humid places on earth, so the only way to stop the spread of COVID is to take precautionary measures.”

Brownstein agreed, saying “while there likely could be an environmental relationship, we can’t count on humidity alone to slow down the epidemic over the summer period.”

IHME, whose coronavirus-projection model is one of the most widely trusted in the field, is now planning to factor in weather-related variables going forward, Mokdad told ABC News.

Professor Mark Urban, an expert in biology at the University of Connecticut, recently authored a paper concluding that ultraviolet light could slow the growth of coronavirus.

“When we look across the US and the world, we find that in places where the ultraviolet light was lowest in the preceding weeks, the COVID-19 growth rate was the highest,” Urban said of his findings, which have yet to be peer reviewed.

Access to fresh, outside air is another possible factor experts are looking at — both because warmer weather sends people outside and because buildings in warmer climates often have more outside air circulating indoors.

“It’s logical that more temperate climates are more likely to have architectural designs that are conducive to outdoor engagement, to free flowing spaces that connect to the outdoors like larger windows, more access to fresh air and more access to daylight,” according to Professor Kevin Van Den Wymelenberg, who runs an institute at the University of Oregon focused on creating buildings that support human health.

Buildings in colder climates are often much “tighter,” Van Den Wymelenberg said, with fewer windows and openings to the outdoors due to extreme weather temperatures and energy codes.

Van Den Wymelenberg said “there is a relationship between outdoor climate and indoor climate but indoor climate is where we should focus our research.”

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