The coronavirus that causes Covid-19, seen in a microscope image, might not survive as readily when hot, humid weather arrives in the Northern Hemisphere this summer.
Photo: /Associated PressAs Covid-19 circles the globe, the most severe outbreaks so far clustered in areas of cool, dry seasonal weather, according to four independent research groups in the U.S., Australia and China that analyzed how temperature and humidity affect the coronavirus that causes the disease.
If their conclusions are borne out, sweltering summer months ahead might offer a lull in new cases across the heavily populated temperate regions of the Northern Hemisphere, they said.
Even so, several of the scientists predicted that the disease would resurge in autumn, when cooler temperatures and low humidity again favor survival and transmission of the SARS-CoV2 coronavirus responsible for the illness.
The four studies—which the research teams themselves posted this month on open-access servers to share the data more quickly—suggest that Covid-19, if unchecked by medical controls, could take its place in a calendar of seasonal epidemics that range from malaria, measles and meningitis to tuberculosis and whooping cough.
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With more data, the effort to link the disease to seasonal weather patterns might eventually help public-health authorities anticipate future outbreaks and marshal timely countermeasures, the U.S.-based scientists said.
“We should prepare for annual or sporadic outbreaks every few years,” said Stephen Kissler, a biomathematician at the Harvard T.H. Chan School of Public Health. He was the lead scientist on a team that developed computer simulations to study scenarios of how the epidemic might spread over the next five years.
These studies rely on data gathered during the epidemic’s first six weeks or so. They are preliminary and haven’t yet been peer-reviewed.
Public-health experts at the World Health Organization and the U.S. Centers for Disease Control and Prevention, who weren’t involved in the studies, say it is too soon to draw conclusions about seasonal behavior of the new coronavirus. While many coronaviruses such as those that cause flu and the common cold are seasonal, medical experts say there isn’t enough evidence to conclude that Covid-19 will go away this summer.
“We don’t know with the Covid-19 virus how it will behave in the warmer weather,” said Andy Pekosz, a microbiologist at the Johns Hopkins University Bloomberg School of Public Health who wasn’t involved in the studies. “Prepare for having to deal with this in the summer months and assume there will be no respite.”
Many diseases follow the seasons. Influenza outbreaks occur each winter. Chickenpox typically peaks in the spring. Polio historically was a scourge of summer. No one is exactly sure why. However, when a new virus first appears among people who have no immunity to it, it may be years before the contagion settles into a predictable pattern, several scientists said.
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“First you get the one big pandemic wave, and then it will start to settle in,” said infectious-disease ecologist Micaela Martinez at Columbia University’s Mailman School of Public Health. “Even if we get a seasonal decline [this summer] in the transmission rate, it could get swamped because there are so many susceptible people.”
While no one is certain, some researchers do believe that hot, steamy weather might strongly affect the survival and transmission rate of the new coronavirus.
In the second of the four new studies, researchers at Beihang University and Tsinghua University in Beijing studied how temperatures and relative humidity affected the natural transmission of Covid-19 in 100 cities across China. They zeroed in on known infections from Jan. 21 through Jan. 23, before China’s authorities intervened to stop its spread.
Funded by the National Natural Science Foundation of China, they calculated that the infection was more contagious in northern China, where temperatures and relative humidity at the time were low, than among cities along the country’s warmer and more humid southeast coast.
“High temperature and high relative humidity significantly reduce the transmission of Covid-19,” they reported. “In July, the arrival of summer and rainy season in the Northern Hemisphere can effectively reduce transmission of the Covid-19; however, risks remain in some countries in the Southern Hemisphere [such as] Australia and South Africa.”
In a preliminary study posted online Friday, public-health analysts at Ausvet, a private epidemiology consulting company based in Australia, also found that higher temperatures appeared to slow transmission of the virus. “The onset of warmer weather in the Northern Hemisphere may modestly reduce rate of spread.” the scientists said.
Researchers led by Dr. Mohammad Sajadi at the Institute of Human Virology at the University of Maryland School of Medicine took a different approach. They mapped outbreaks of the disease to see how temperature and specific humidity, which measures the absolute amount of water vapor in a volume of air, affected where cases of the disease developed. Their work, initially posted online, is being submitted for publication.
Dr. Sajadi and his colleagues found that the most severe outbreaks clustered in a narrow band of consistently similar weather across the Northern Hemisphere between 30 degrees latitude and 50 degrees latitude North, running through China to South Korea, Japan, Iran, Italy, France and the U.S. Pacific Northwest.
Temperatures in the zone stayed between 5 and 11 degrees Celsius (41 to 51 degrees Fahrenheit). Relative humidity was between 44% and 84%, with low specific and absolute humidity, the scientists said.
“It couldn’t have been bad luck that these particular places were hit,” said Dr. Sajadi. “This virus is acting like a seasonal respiratory virus. We could be wrong, but with the data we have, we think that is the most likely scenario.”
Using weather modeling, it may be possible to predict regions likely to be at higher risk of significant community spread of Covid-19 in the coming weeks, he said.
“People want to know if this is going to go away in the summer,” Dr. Sajadi said. “Seasonal respiratory viruses never really go away. They just go to different areas.”
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