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Why Coronavirus Superspreading Events Happen


Researchers now think the main way the coronavirus spreads is through groups scenarios - the indoor party, the business conference, a cruise ship. This is not typically how a virus like the flu spreads, but it seems to be a distinguishing trait of the coronavirus. NPR's Pien Huang explains why.

PIEN HUANG, BYLINE: A person with a high-viral load walks into a bar, and if that bar was in Hong Kong's Lan Kwai Fong nightlife district this spring, that person may have triggered one of the biggest coronavirus clusters in the city. The virus spread between 106 bargoers, bar workers and musicians in that district over a period of two weeks in March.

GABRIEL LEUNG: And when these clusters get very big, then you sort of then call it superspreading.

HUANG: Gabriel Leung is dean of medicine at the University of Hong Kong and co-author on a forthcoming paper that looks at superspreading events in Hong Kong. He says that bar district had all the high-risk factors that make it easy for the coronavirus to spread.

LEUNG: Any setting that is an enclosed space, that is poorly ventilated, that is crowded and that has unprotected behavior, that would tend to create a lot of clusters.

HUANG: Maria Van Kerkhove, a top scientists with the World Health Organization, says this pattern of spread is a hallmark of the coronavirus.

MARIA VAN KERKHOVE: There's some really good estimates out there that suggest that between 10% and 20% of cases are responsible for about 80% of transmission events.

HUANG: This pattern where one person or one event results in a lot of cases was also seen with the original SARS virus, which swept through Asia in 2003. Jamie Lloyd-Smith at UCLA studies how diseases spread. He's looked at a wide range of past outbreaks, from measles to smallpox to pneumonic plague. And in all cases, he says, there are some clusters of transmission, but for the coronavirus and its cousin virus SARS1, clusters seem to be a major driver.

JAMIE LLOYD-SMITH: Both SARS1 and SARS2 are - they're relatively superspready (ph), extreme.

HUANG: One difference between these viruses is that this coronavirus is more of a party-crashing virus. It spreads pretty efficiently from people who don't know they're sick. Leung at the University of Hong Kong says that's different from flu or even the first SARS coronavirus.

LEUNG: Forty percent of transmission will have taken place before the index patient even shows symptoms.

HUANG: Research is showing that people may actually be most contagious in the day or two before they start feeling sick, which is why we're seeing so many superspreading events in bars and nightclubs and restaurants and not just in hospitals. It's hitching rides from one person's respiratory tract to another's, all while everyone is feeling totally fine. Seema Lakdawala, a flu researcher at the University of Pittsburgh, says that some people may also naturally spread the virus more than others. It may come down to differences in biology, like how much virus the person sheds or even how it comes out. One theory at the moment, she says...

SEEMA LAKDAWALA: It could be that people who have really sticky mucus are more likely to be superspreaders.

HUANG: Scientists are working hard to untangle the details of why superspreading events are happening, but the bones of it are clear - when a person with a high-viral load walks into a bar, be careful.

Pien Huang, NPR News.

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

Pien Huang is a health reporter on the Science desk. She was NPR's first Reflect America Fellow, working with shows, desks and podcasts to bring more diverse voices to air and online.
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