Most People With Coronavirus Won’t Spread It. Why Do a Few Infect Many?
Growing evidence shows most infected people aren’t spreading the virus. But whether you become a superspreader probably depends more on circumstance than biology.
If Covid-19 was like the flu, you’d expect the outbreaks in different places to be mostly the same size. But Dr. Kucharski and his colleagues found a wide variation. The best way to explain this pattern, they found, was that 10 percent of infected people were responsible for 80 percent of new infections. Which meant that most people passed on the virus to few, if any, others.
Researchers found many superspreading events. Just 2 percent of people were responsible for 20 percent of transmissions. Now researchers are trying to figure out why so few people spread the virus to so many. They’re trying to answer three questions: Who are the superspreaders? When does superspreading take place? And where?
As for the first question, doctors have observed that viruses can multiply to bigger numbers inside some people than others. It’s possible that some people become virus chimneys, blasting out clouds of pathogens with each breath.
Some people also have more opportunity to get sick, and to then make other people sick. A bus driver or a nursing home worker may sit at a hub in the social network, while most people are less likely to come into contact with others — especially in a lockdown.
A lot of transmission seems to happen in a narrow window of time starting a couple days after infection, even before symptoms emerge. If people aren’t around a lot of people during that window, they can’t pass it along.
And certain places seem to lend themselves to superspreading. A busy bar, for example, is full of people talking loudly. Any one of them could spew out viruses without ever coughing. And without good ventilation, the viruses can linger in the air for hours.
A study from Japan this month found clusters of coronavirus cases in health care facilities, nursing homes, day care centers, restaurants, bars, workplaces, and musical events such as live concerts and karaoke parties.
This pattern of superspreading could explain the puzzling lag in Italy between the arrival of the virus and the rise of the epidemic. And geneticists have found a similar lag in other countries: The first viruses to crop up in a given region don’t give rise to the epidemics that come weeks later.
Many countries and states have fought outbreaks with lockdowns, which have managed to draw down Covid-19’s reproductive number. But as governments move toward reopening, they shouldn’t get complacent and forget the virus’s potential for superspreading.
“You can really go from thinking you’ve got things under control to having an out-of-control outbreak in a matter of a week,” Dr. Lloyd-Smith said.
Singapore’s health authorities earned praise early on for holding down the epidemic by carefully tracing cases of Covid-19. But they didn’t appreciate that huge dormitories where migrant workers lived were prime spots for superspreading events. Now they are wrestling with a resurgence of the virus.
Since most transmission happens only in a small number of similar situations, it may be possible to come up with smart strategies to stop them from happening. It may be possible to avoid crippling, across-the-board lockdowns by targeting the superspreading events.
“By curbing the activities in quite a small proportion of our life, we could actually reduce most of the risk,” said Dr. Kucharski.