New research suggests heat, humidity could put a damper on coronavirus

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Could Florida's heat and humidity stop the new coronvirus dead in its tracks, or at least slow its spread significantly?

Scientists say there is hope, fueled by some recent research, but it’s hard to say with a new virus. How strong a grip coronavirus gets on Florida and other hot and humid climates will depend on the balance between how fast the virus spreads against any tempering effects of heat and humidity, scientists say. But it also depends on whether such conditions truly hamper this specific virus, as they do influenza viruses.

"We don’t know that yet," Dr. Lynn B. “Marty” Martin, a professor at the University of South Florida College of Public Health, said via email.

Martin researches disease ecology. He recently featured on his podcast John Drake, director of the Center for the Ecology of Infectious Diseases at the University of Georgia. Drake has been working with the Centers for Disease Control to understand the dynamics of the COVID-19 outbreak and to identify ways to slow its spread.

"I think it's really premature to speculate about that," Drake said when asked how widely the virus could spread among the American population.

Some recent research suggests half or more of Americans could eventually get infected.

German Chancellor Angela Merkel recently warned that up to 70% of people in her country could contract the virus.

More: Florida coronavirus updates: Agriculture Commissioner asks DeSantis to issue 'stay-at-home' order

But one study, published this month by Chinese researchers at Beihang University and Tsinghua University, showed spread of the virus could be tempered in warmer, more humid climates.

The researchers found that, "high temperature and high relative humidity significantly reduce the transmission of COVID-19, respectively, even after controlling for population density and GDP per capita of cities," the researchers wrote in the paper, published in SSRN, formerly the Social Science Research Network, an international journal.

They looked at the reproductive number — the number of cases, on average, an infected person will cause while contagious — for 100 Chinese cities.

Every 1 degree Celsius increase in temperature and 1% increase in relative humidity lowered the reproductive number of the virus by 0.0383 and 0.0224, respectively, the researchers found.

Their findings are consistent with other research that shows high temperature and high humidity significantly reduce the transmission of influenza.

"It indicates that the arrival of summer and rainy season in the northern hemisphere can effectively reduce the transmission of the COVID-19," the researchers wrote in the paper.

The researchers point to the fact that flu viruses are more stable in cold temperatures, and the respiratory droplets that act as viral containers, remain airborne longer in dry air. Also, cold, dry weather weakens immunity. As humidity increases, so does the size of viral droplets, which fall from the air faster, providing less opportunity for others to breathe in the infectious droplets.

"These mechanisms are also likely to apply to the COVID-19 transmission. Our result is also consistent with the evidence that high temperature and high relative humidity reduce the viability of SARS coronavirus."

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Another concern, however, is that the new virus is stable for several hours to days in aerosols and on surfaces, according to another new study, this one by the National Institutes of Health, CDC, UCLA and Princeton University scientists.

The study, published in the New England Journal of Medicine, found that the new coronavirus was detectable for up to three hours in aerosols, up to four hours on copper, and up to 24 hours on cardboard and two to three days on plastic and stainless steel.

But scientists point with further hope to other studies the prove heat and humidity puts a damper on the spread of influenza viruses.

Low humidity, to the contrary, increases spread of those viruses. An animal study of influenza last year by Yale researchers found that low humidity obstructs immune response by preventing the tiny hair-like structures, or cilia, in airways cells from removing viruses and mucus. It also limited those cells' repair functions. Dry air also hampered the ability of infected cells to trigger the immune system to attack.

Another ray of hope for hot, humid Florida shines from the nations with warmer climates that had imported novel coronavirus cases, such as Thailand, Vietnam and Singapore. Those countries have so far been spared large COVID-19 outbreaks, said Dr. Yang Yang, an associate professor at University of Florida's department of biostatistics.

But Yang also warned of a lack of sufficient scientific studies to prove definitively that warmer temperatures and high humidity temper this new virus.

Thailand, Vietnam and Singapore also enacted much more extreme measures than in America, including faster lock-downs and much more testing to identify coronavirus hot spots early, to stamp out further infections.

"It's really difficult to stop it at an early stage, because that requires a very active screening process," Yang said of of why China failed to contain the new virus.

Wuhan, China, the epicenter of the new coronavirus outbreak, is a large hub connecting all regions of China via rail and a major international airport. Availability of connecting flights, the timing of the outbreak during the Chinese (Lunar) New Year, and Wuhan's massive rail hub allowed the virus to spread throughout China, and eventually, the globe, scientists say.

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Jim Waymer is environment reporter at FLORIDA TODAY.

Contact Waymer at 321-242-3663

or jwaymer@floridatoday.com.

Twitter: @JWayEnviro

Facebook: www.facebook.com/jim.waymer

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