70% of people infected with the coronavirus did not pass it to anyone, preliminary research shows. Superspreading events account for most transmission.
An average person with the coronavirus infects about two other people, but an infected person sometimes passes the virus to far more people during a superspreader event. New research suggests such events, which typically involve indoor gatherings, are responsible for most coronavirus transmission. Experts found that just 20% of coronavirus cases resulted in 80% of transmissions. An estimated 70% of infected patients studied didn't pass the virus at all. Countries may be able to avoid more lockdowns during future waves of infections by targeting locations and activities that beget superspreading. Visit Business Insider's homepage for more stories.
Superspreader events, in which one person infects a disproportionately large number of others, are the primary means by which the coronavirus spreads, new research suggests. A group of epidemiologists in Hong Kong found that just 20% of cases studied there were responsible for 80% of all coronavirus transmissions. The researchers also found 70% of people infected with the coronavirus didn't pass it to anyone else and that all superspreading events involved indoor social gatherings. "That's the picture we have so far," Ben Cowling, one of the study coauthors, told Business Insider. "Superspreading events are happening more than we expected, more than what could be explained by chance. The frequency of superspreading is beyond what we could have imagined." That information could inform how policymakers craft rules to keep people safe. "Now we know which measures might give you the most bang for your buck — if we could stop the superspreading from happening, we'd benefit the most people," Cowling said. A small number of superspreading events account for most transmission Superspreader events around the world have created clusters of coronavirus infections that cropped up almost overnight. A South Korean churchgoer infected 43 others in February, a singer infected 53 people at a choir practice in Washington a month later, and a New York lawyer was responsible for passing the coronavirus to more than 100 others in his community.
For their research, Cowling and his colleagues examined more than 1,000 coronavirus cases in Hong Kong between January 23 and April 28. They found that superspreading was the primary means of transmission in the city. About 350 of the cases analyzed were a result of community spread, while the rest were imported from other countries. Within the community-spread cases, more than half were connected to six superspreading events. The term "superspreader" refers to an infected person who transmits the virus to more people than a typical infected person would. A virus' R0 value (pronounced "R-naught") refers to the average number of people that one sick person goes on to infect in a group with no immunity. The R0 of the coronavirus so far seems to hover between 2 and 2.5. But in the case of these Hong Kong superspreading events, one person infected at least three times that many people. In fact, 20% of cases caused 80% of transmissions, a majority of which were linked to superspreading events at a wedding, temple, and multiple bars in the city's Lan Kwai Fong district. The remaining 20% of transmissions were result of another just 10% of cases, when infected patients passed along the virus to one, or at most two, other people — generally someone in their households. "Social exposures produced a greater number of secondary cases compared to family or work exposures," the study authors wrote, adding that reducing superspreading events could have a considerable effect in lowering the virus' R0. The 80-20 rule In a New York Times article about his team's study, which has yet to be peer-reviewed, Cowling wrote: "You might be wondering if our study, or the experience of Hong Kong, with its small number of total infections, is more broadly representative. We think so."
Indeed, other research supports his findings: A 2011 study found that 20% of a population was responsible for 80% of the transmissions of many diseases, including malaria. This is known as the "80-20 rule." Some scientists think that the ratio could be even smaller. A model from researchers at the London School of Hygiene and Tropical Medicine suggested only 10% of coronavirus cases accounted for 80% of global transmissions. Preliminary research that examined more than 200 coronavirus cases in Israel found that between 1 and 10% of cases were linked to 80% of transmissions. Another study from Shenzhen, China, yielded a similar conclusion: Between 8 and 9% of cases caused 80% of transmissions. Superspreading events happen in crowded indoor areas Coronavirus superspreader events have shared a few key characteristics: They've involved indoor gatherings in which a lot of people from different households were in close, extended contact. For example, a superspreader event in Arkansas involved a pastor and his wife who attended church events and a Bible study group a few days before they developed coronavirus symptoms. Of the 92 people they came into contact with, 35 got sick. Seven had to be hospitalized. Three died.
Offices and restaurants can be infection hot spots too. A study of an outbreak in a call center in Seoul, South Korea, showed that almost half the employees on one floor got infected. Nearly all of them sat in the same section. In that sense, it's not that certain individual people are more contagious than others or shed more virus. Instead, there's a type of activity that gives people access to a greater number of people in areas conducive to the virus' spread, Cowling said. Research has found time and again that the risk of coronavirus transmission is higher indoors in poorly ventilated spaces where lots of people have sustained contact. "You can't have a superspreading event unless there are a lot of people around, so you have to be very careful still about gatherings of people of any size — that includes religious services," William Schaffner, an infectious-disease expert at Vanderbilt University, previously told Business Insider. If we target gatherings that could become superspreader events, we could avoid more lockdowns Cowling said the study findings could inform countries' responses to future waves of coronavirus infections. "We'll be in a much better position to deal with the second wave this fall," he said. "This knowledge gives us the chance to take more measured actions without going into full lockdown again."
A few countries, like Japan and South Korea, have already shown that it's possible to ride out an outbreak without dramatically restricting citizens' movements or shutting down all stores, restaurants, and schools. Japan's success stems from adherence to the "3 C's rule." The government told people to avoid closed spaces, crowded places, and close-contact settings — all of which are ripe for superspreading events. Going forward, Cowling thinks other countries could benefit from instituting rules that target the source of most transmissions (in addition to continued contact tracing and testing), rather than blanket shelter-in-place orders.
"Anything outdoors is fine. I'm less concerned about protests," he said, adding that restaurants and bars could also probably operate at 50% capacity, with empty tables between diners. "We need to figure out how many people per square meter is acceptable," Cowling said. "Meetings and religious activities could go on, but with reduced numbers of people." SEE ALSO: Coronavirus super-spreader events all have notable similarities — and they reveal the types of gatherings we should avoid for years Join the conversation about this story » NOW WATCH: How location data can help track and stop the spread of COVID-19
More like this (3)
A Maine wedding is linked to 147 coronavirus cases and 3 deaths. Infections spilled over into a jail and two nursing homes.
Summary List Placement At the start of August, Maine had emerged from the coronavirus pandemic relatively...Summary List Placement At the start of August, Maine had emerged from the coronavirus pandemic relatively unscathed. The state had one of the lowest case tallies in the US — and its average number of new daily cases was declining. That was before an indoor church wedding and reception on August 7 ignited a chain of infections across the state. As of Saturday, 147 coronavirus cases were linked to the wedding — including three deaths. One of the deceased victims, an 83-year-old woman, died on August 31. Her husband, a 97-year-old World War II veteran, was hospitalized with the virus but is scheduled to be discharged soon. None of the three people who died were wedding attendees. The infections spread from Millinocket, a town in north-central Maine, to two nursing homes and a jail about 230 miles south. Maine's weekly average of coronavirus cases has now risen to 27 new daily cases, compared to 15 new daily cases on the day of the wedding. "One of the things we've learned over the past six months of working with outbreaks and COVID-19 is that no outbreak is an island," Maine CDC Director Dr. Nirav Shah said during a Thursday press briefing. "What this really hammers home is that outbreaks are not isolated events. One outbreak can quickly lead to several more outbreaks, especially in a close geographic area." Maine's contract tracers linked 72 wedding-related cases to the York County Jail, including 46 infections among inmates. One of the jail employees who tested positive was a guest at the wedding. The local sheriff, William King, said guards and inmates at the jail weren't required to wear masks prior to the outbreak. Local officials also linked the wedding to an outbreak at Maplecrest Rehabilitation & Living Center in Madison, Maine. One of the wedding guests had contact with a staff member at the nursing facility, who spread the infection to eight residents and seven staffers. Contract tracers also recorded 10 cases at the Calvary Baptist Church in Sanford, Maine. The pastor at that church, Todd Bell, officiated the Millinocket wedding. At a sermon on August 30, Bell said people should have the liberty to choose whether to wear a face mask. "I'll tell you what the world wants all the churches to do," Bell said. "They want us to shut down, go home, and let people get used to that just long enough until we can finally stop the advancing of the Gospel." The Maine wedding defied local laws The Millinocket wedding is a classic example of a superspreading event, in which one person infects a disproportionately large number of people. Researchers in Hong Kong have suggested that superspreader events involving indoor social gatherings may be responsible for the majority of coronavirus transmission. Weddings are particularly high-risk, according to the Centers for Disease Control and Prevention, because attendees often travel from outside the local area, which brings a risk of either introducing the coronavirus to local guests or spreading the virus to other parts of the state or country. A study from Japanese scientists estimated that the odds of a person spreading the coronavirus in a closed environment is almost 19 times higher than in an open-air environment, though that research is still awaiting peer review. Maine's current limit on indoor gatherings is 50 people. At least 65 people attended the wedding ceremony at the Tri-Town Baptist Church in Millinocket. The reception at the Big Moose Inn also exceeded the state's indoor limit. Disobeying local ordinances for public gatherings has had detrimental consequences in other parts of the US as well. In July, a San Francisco church attempted to hold an indoor wedding, despite the fact that the city requires religious services to be held outdoors. At least 10 attendees, including the newlywed couple, later tested positive for the coronavirus.SEE ALSO: A San Francisco couple held a secret wedding against city rules. Then they tested positive for the coronavirus. Join the conversation about this story » NOW WATCH: What's next for dating during coronavirus, according to an anthropologist, pathogen expert, and love-life coach
Growing evidence shows most infected people aren’t spreading the virus. But whether you become a superspreader...Growing evidence shows most infected people aren’t spreading the virus. But whether you become a superspreader probably depends more on circumstance than biology.