“It’s a combination of host susceptibility and viral stability,” Dr. Hazhir Rahmandad, a co-author of the study from the Massachusetts Institute of Technology, suggested a high UV index could drive people indoors, where it’s easier to spread the virus. Regions with more ultraviolet light, like Colombia and Ecuador, were associated with higher transmission rates in the study, even though it’s believed that ultraviolet light can destroy the virus. Some differences may be driven by how people react to the weather. And dry air helps respiratory viruses in droplets expelled from a person’s nose or mouth stay in the air longer, making them more likely to infect someone else. Scientists have found evidence that this coronavirus dies in warmer weather as well. Research has also shown that the hotter it is, the faster some viruses will break down outside of a human host. It’s not fully understood how warm weather controls the spread of the virus.Īkiko Iwasaki, an immunologist at Yale University who wasn’t affiliated with this study, said that people are more susceptible to infections in other respiratory viruses when the air they breathe is cold or particularly dry. But in most places, even that level of intervention will allow infections to rise again by fall and winter. If every state pushes to decrease transmission by 60%, the entire country will see a reprieve during the summer. Rather than relying on weather alone to slow the spread, it’s possible that warm temperatures will work alongside other interventions to adequately slow the virus. “Possibly we’ll see slower spread in the summer, but even that summer spread could seed the virus in a quiet way, and then come winter it would be a bigger problem than it would have been.” “Basing policy decisions on the assumption of seasonality is dangerous,” said Anice Lowen, a professor of microbiology and immunology at Emory University, which was not affiliated with this study. The warm weather effects could help justify those decisions in the short term, as governors point to falling infections as a sign that the virus is defeated. Headed into summer, governors have already signaled their eagerness to give up social distancing and reopen their economies before passing the White House’s own guidelines for testing and new infections. But by winter, those benefits likewise fade, when cold weather could boost transmissions slightly. In New York City, weather could reduce the rate of infections by a quarter during the hottest months. This value can vary from place to place and does not take into account the effect of actions like social distancing nor a rise in the population that’s immune to the virus. Note: Assumes a reproductive number of 2.6, meaning one infected person will, on average, infect 2.6 other people. Here is how those predictions look in every U.S. But all the evidence available suggests weather plays a role with coronavirus transmission, but only a small one. Researchers will continue to debate the measurements and methodology others may come up with alternative projections. Like all weather forecasts, there is significant uncertainty about the results. In parts of India and Pakistan, conditions during the hotter months could make the virus less than half as likely to infect new hosts.įor regions facing cold, bitter winters, weather has the opposite effect, increasing the rate of infections and making it even harder to control the virus. In some of the hottest cities in the United States, like Phoenix, high temperatures could drive down the rate of infections by over 40 percent. The study, conducted by researchers at six academic institutions, found that warm weather could play a small role in slowing the virus in at least a few places and for a few months. “No government should rely on the effect of the weather.” “At the end of the day, this whole effect from weather and pollution is still pretty minor,” said Mohammad Jalali, an assistant professor at Harvard Medical School and one of the study’s authors. Without social distancing and other interventions, summer will offer only a modest respite in some places, meaning stay-at-home orders and other government interventions will most likely need to continue throughout the summer. Shows metropolitan areas and cities with more than 500,000 residents.
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