ARI SHAPIRO, HOST:
One of the many open questions about the new coronavirus is whether the season will affect its transmission. The worst outbreaks so far have been in colder parts of the Northern Hemisphere during winter or early spring. Will warmer weather slow the transmission? NPR's Jason Beaubien reports this is a key question about COVID-19 that scientists are trying to answer.
JASON BEAUBIEN, BYLINE: If you want to understand the seasonality issue around this new virus, you have to understand how this virus spreads, and unfortunately, that still isn't entirely clear. There are two main theories. One theory would support that the coronavirus would come and go with the seasons; the other does not. Let's start with the airborne theory. This one suggests seasonality. Akiko Iwasaki is a professor of immunobiology at the Yale University School of Medicine. She says it's been recognized for hundreds of years that people get sick in the winter.
AKIKO IWASAKI: During the winter months, we tend to have a surge in the cases of respiratory infection, including the influenza viruses.
BEAUBIEN: There are a number of reasons why, but Iwasaki says the main driver of this is that changes in the amount of water vapor in the air in winter make it easier for viruses to become airborne.
IWASAKI: When you cough or sneeze or even talk, you're generating these droplets that's coming out of your mouth, and some of that, if you're infected, will contain virus particles.
BEAUBIEN: And if those virus particles go out into dry, cold air, it's easier for them to linger.
IWASAKI: And in very arid conditions, those particles lose the water vapor and they become airborne, and so they can persist in the air for a very long time.
BEAUBIEN: Now, she's talking about traditional cold and flu viruses that have been studied for years. The question is whether the new coronavirus will also behave in this way. Iwasaki expects it will.
IWASAKI: The property of the virus is likely similar to the common cold version of the virus. And so I would expect that this coronavirus can also stay in the air better at lower relative humidity, meaning the indoor conditions that you find in the winter months.
BEAUBIEN: So if this virus depends on this airborne route, we would expect to see transmission slow or even stop in the summer, when the air is warmer and carries more moisture. And there's research to back this up. In laboratories, scientists have seen that temperature and humidity affect how long the new coronavirus can hang in the air.
The other main theory of transmission is that the virus is spread on large droplets. The WHO and the CDC based their guidelines for frequent hand-washing and 6 feet of social distance on this. The idea is that the virus is riding on a particle that flies out of someone's nose or mouth when they sneeze or cough; the spittle doesn't go very far, then drops out of the air onto surfaces. Under this model - let's say a person with COVID-19 is eating in a restaurant. They're far more likely to infect someone at their own table than someone across the room. If the new coronavirus primarily spreads this way, on large droplets, seasonality may not matter.
But some researchers caution against assuming that any new virus, regardless of how it spreads, will follow a seasonal pattern.
ANICE LOWEN: A new virus coming into the population - everybody is susceptible.
BEAUBIEN: Anice Lowen is an associate professor of microbiology and immunology at Emory University. She says, with a new virus, no one has had a chance to develop immunity.
LOWEN: Everybody is sort of fair game for an infection to that virus. So it makes the virus transmit much more readily, such that it may continue to transmit even when the climactic conditions of the season is not optimal for transmission.
BEAUBIEN: She says this virus may well continue to spread straight through the summer here in the U.S. Lowen points out that when the H1N1 influenza virus emerged in 2009, it didn't start in winter, the way the seasonal flu does.
LOWEN: The 2009 pandemic started in April and May, well outside the normal seasonality.
BEAUBIEN: Eventually, the H1N1 virus did fall into a more traditional seasonal pattern, but it took several years. As the Northern Hemisphere moves into summer, researchers will be watching closely whether transmission of the new coronavirus drops as the temperature goes up.
Jason Beaubien, NPR News.
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