But SARS-CoV-2, like two of its cousins, severe acute respiratory syndrome (SARS) and Middle East respiratory syndrome (MERS), seems especially prone to attacking groups of tightly connected people while sparing others. It’s an encouraging finding, scientists say, because it suggests that restricting gatherings where superspreading is likely to occur will have a major impact on transmission, and that other restrictions—on outdoor activity, for example—might be eased.
But superspreading events are ill-understood and difficult to study, and the findings can lead to heartbreak and fear of stigma in patients who touch them off.In January, Julien Riou and Christian Althaus at the University of Bern simulated the epidemic in China for different combinations of R and k and compared the outcomes with what had actually taken place. (Japan, which was hit early but has kept the epidemic under control, has built its COVID-19 strategy explicitly around avoiding clusters, advising citizens to avoid closed spaces and crowded conditions.) Entering a high-risk setting in that period may touch off a superspreading event, Kucharski says; “Two days later, that person could behave in the same way and you wouldn’t see the same outcome.” After South Korea relaxed social distancing rules in early May, a man who later tested positive for COVID-19 visited several clubs in Seoul; public health officials scrambled to identify thousands of potential contacts and have already found 170 new cases.
“You’re basically saying: We don’t know enough about where transmission is happening to be able to target it, so we’re just going to target all of it.” Fraser, who is tracking HIV transmission in Africa by sequencing virus isolates, says it is a difficult trade-off, but one that can be managed through good oversight and engagement with communities. Epidemiologists have “a duty” to study clusters, he says: “Understanding these processes is going to improve infection control, and that’s going to improve all of our lives.”