Will Summer Slow The Spread Of COVID-19? Scientists Try To Figure It Out | WGLT

Will Summer Slow The Spread Of COVID-19? Scientists Try To Figure It Out

Apr 9, 2020
Originally published on April 10, 2020 10:23 am

The worst outbreaks of COVID-19 so far have been in colder parts of the Northern Hemisphere during winter or early spring. Will warmer weather slow the transmission?

Could the Southern Hemisphere see outbreaks intensify as that part of the globe moves into winter?

And is it possible that transmission might be naturally interrupted as it is each year for the seasonal flu?

These are some of the key questions about COVID-19 that scientists are trying to answer.

"The seasonality of flu and other respiratory viruses has been known for hundreds of years, if not thousands," says Akiko Iwasaki, a professor of immunobiology at the Yale University School of Medicine. "During the winter months, we tend to have a surge in the cases of respiratory infection, including the influenza viruses."

There are a number of factors that contribute to why the flu arrives in the winter, but Iwasaki says the primary factor is the relative humidity of the air. In winter, the drop in the amount of water vapor in the cold, dry air makes it easier for viruses to become airborne.

This makes what Iwasaki calls the "perfect setting" for respiratory viruses to transmit.

"When you cough or sneeze or even talk, you're generating these droplets that are coming out of your mouth," she says. "And some of them, if you're infected, will contain virus particles. In very arid conditions, those particles lose the water vapor and they become airborne." This allows the virus to persist in the air for a long time, much longer than in summer.

Of course, 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 this way.

Iwasaki thinks so.

"This novel coronavirus is a cousin of the coronaviruses that are circulating in humans and just causing the common cold," she says. "So the property of the virus is likely to be similar to the common cold version of the coronavirus. So I would expect that this [new] coronavirus can also stay in the air better at lower relative humidity, meaning the indoor conditions that you find in the winter months." Meaning that it would be more likely to spread in winter.

But for this to happen, there has to be airborne transmission.

The World Health Organization and the U.S. Centers for Disease Control and Prevention in their guidance on SARS-CoV2 downplay airborne spread, saying instead that the primary form of transmission is by "large respiratory droplets."This debate – airborne vs. droplets — is a crucial divergence in thought when it comes to figuring out if COVID-19 is going to be seasonal. If the primary form of transmission is airborne, then the novel coronavirus could become a seasonal disease. If it mainly spreads through "large respiratory droplets", then seasonality is less likely.

There's evidence pointing toward both possible transmission routes.

The transmission through "large respiratory droplets" works this way. When an infected person sneezes or coughs, virus-laden spittle flies out into the air. The particles, which may even be big enough to see, don't go very far: WHO estimates they travel about 3 feet before dropping onto surfaces directly around the infected person.

According to this model, if a person with COVID-19 is eating at restaurant, they're far more likely to infect someone at their table than someone across the room.

So if the new coronavirus primarily spreads this way on large droplets, seasonality may not matter.

Studies out of Asia have documented this type of spread among people in close contact to one another.

And in 2003, another deadly coronavirus that's closely linked to SARS-CoV 2 —the offical name of the current coronavirus — severe acute respiratory syndrome is believed to have spread primarily through large droplets.

But researchers in the U.S. have aerosolized the novel coronavirus in laboratory settings – mechanically spraying virus into the air so that it floats. They found it can remain in the air for more than an hour. At certain temperatures and humidity levels, they've seen in preliminary results that aerosolized particles can remain suspended in the air even longer than some flu viruses.

WHO agrees that the new coronavirus can be aerosolized, especially during some invasive medical procedures, but the global health body insists this isn't a primary form of transmission.

Iwasaki disagrees.

"I would be quite surprised actually, if this novel coronavirus isn't aerosolized," she says. "Why would it be different from other types of respiratory viruses?"

But other researchers caution against assuming that any new virus, regardless of how it spreads, will follow a seasonal pattern.

"With a new virus coming into the population, everybody is susceptible," says Anice Lowen, an associate professor of microbiology and immunology at Emory University. She notes that no one has had a chance to develop immunity: "Everybody is sort of fair game for an infection [from] that virus. So it makes the virus transmit much more readily, such that it may continue to transmit even when the climactic conditions ... or the season ... is not optimal for transmission."

She says this new coronavirus 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.

"The 2009 pandemic started in April and May," she says, "well outside the normal seasonality."

Eventually the H1N1 virus did fall into a more traditional seasonal pattern, but it took several years.

Ben Cowling, an epidemiologist at the University of Hong Kong, also says higher temperatures haven't seemed to have stopped the spread of this virus.

"What we've seen in Southeast Asia is that there was heavy seeding of infections [from China] into Thailand, into Malaysia, Indonesia and some other places which are relatively hotter," he says. "And it looks at the moment like spread of infection has been a little bit slower in some of the hotter countries than in the cooler countries. So my interpretation is there's a little bit of seasonal pressure, perhaps reducing transmission by 10 percent, but that's not nearly enough to stop an epidemic from occurring when we have such a high basic transmissibility."

Even in the U.S. Southern states like Florida and Louisiana have been hard-hit along with colder states like New York and Michigan.

But the worst outbreaks so far globally have been in cooler locations. So as the Southern Hemisphere moves into winter, researchers will be watching closely whether transmission of SARS-CoV 2 increases in South America, Africa and Oceana as the temperature drops.

Copyright 2020 NPR. To see more, visit https://www.npr.org.

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.

(SOUNDBITE OF MUSIC) Transcript provided by NPR, Copyright NPR.