The novel coronavirus appears to be seasonal in nature, with major outbreaks occurring mainly in regions that match a specific set of climate conditions, a new study argues.
All areas experiencing significant outbreaks of COVID-19 fall within a northern corridor that has an average temperature of 41 to 52 degrees Fahrenheit and an average humidity of 47 percent to 79 percent, according to virology researchers.
These affected regions -- China, South Korea, Japan, Iran, Northern Italy, Seattle and Northern California -- all fall within a band between 30 to 50 degrees Northern latitude. There's been a lack of significant spread of COVID-19 into countries farther South.
"To us, this suggests temperature and also low absolute and specific humidity could hold a key role in transmission," said lead researcher Dr. Mohammad Sajadi, an associate professor of medicine with the Institute of Human Virology at the University of Maryland School of Medicine, in Baltimore.
"Putting all this together, we think the distribution of significant community outbreaks along restricted latitude, temperature and humidity are consistent with the behavior of a seasonal respiratory virus," Sajadi continued.
This doesn't mean that COVID-19 infection rates can be expected to fall with the coming of summer, however.
Infectious disease experts note that the novel coronavirus has proven particularly infectious, given that humans have no established immunity against it.
The coronavirus has an estimated transmission rate of 2.5 or higher, said Elizabeth Halloran, a professor of biostatistics at the University of Washington School of Public Health, in Seattle. That means every two people infected with the virus will likely spread it to a total five more people.
A virus stops being contagious when its transmission rate drops below 1, meaning that a person infected with it is not likely to spread it to another human being.
"It's going to be difficult, even if it does go down somewhat seasonally in the summer, to bring that down necessarily below 1," Halloran said. "We're looking at a very contagious infection."
For this study, virologists analyzed major outbreaks of COVID-19 and tracked the specific weather conditions in those regions.
The investigators found that in cities where the coronavirus is spreading within a community -- Wuhan, Milan and Tokyo -- temperatures did not drop below the freezing mark.
Lab studies also showed that a temperature of 39 degrees Fahrenheit and a humidity level of 20 percent to 80 percent is most conducive to the virus' survival.
"Based on what we have documented so far, it appears that the virus has a harder time spreading between people in warmer, tropical climates," Sajadi said.
But Sajadi and his colleagues warned that risk of community spread could increase in more northern areas like the Mid-Atlantic states and New England as spring blooms.
"We have a testable hypothesis that requires more research to confirm," Sajadi said. "If we do confirm this with further studies, it indicates that we may want to use the data for more targeted health system preparation, surveillance and containment efforts."
No one's really sure why season is a factor in the spread of viruses like influenza and coronavirus, experts said. It's not been established whether viruses can't survive in warmer weather, or if warmer climes somehow interfere with their ability to spread between people.
Further, each virus responds to weather in its own way, noted Dr. Martin Hirsch, a professor of infectious diseases and immunology at the Harvard T.H. Chan School of Public Health, in Boston.
"SARS [severe acute respiratory syndrome] appeared in winter and was gone by June. Others like MERS [Middle East respiratory syndrome] certainly persist on the Arabian peninsula, however, which is pretty hot," Hirsch noted.
This sort of predictive modeling "will be very important to ongoing efforts to understand novel coronavirus and mitigate its effects," said Dr. Michael Grosso, chief medical officer at Huntington Hospital in New York. "Needless to say, it would be reassuring to know that virus activity will wane with warmer weather."
But public health experts expect more will be needed than a change in season to stop the spread of COVID-19.
"Environmental conditions are one of many things that play a role within disease transmission as it is," said Nicholas DeFelice, an assistant professor of environmental medicine and public health with the Icahn School of Medicine at Mount Sinai in New York City. "It's hard to say this is the driver of it, when most likely everybody's susceptible to this new virus and that's what's driving these outbreaks. If people are susceptible, the virus can still transmit even under less-than-ideal circumstances."
"As the entire population probably has no previous immunity to this novel virus, it may not initially act like what we think as a seasonal respiratory virus. Also, keep in mind that being in a low-risk area does not necessarily mean that a significant outbreak will not happen there," Sajadi said.
"Public health measures may play the strongest predictive role in determining whether this virus spreads widely in the U.S.," Sajadi continued. "That is why implementation of social distancing is just as crucial in Miami as it is in New York, despite the differences in temperature."
The new study was published online on the open-data site SSRN.
The U.S. Centers for Disease Control and Prevention has more about COVID-19.
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