If the Chinese epidemic was a big fire that sent sparks flying around the world, most of the sparks simply fizzled out. If k is really 0.1, then most chains of infection die out by themselves and SARS-CoV-2 needs to be introduced undetected into a new country at least four times to have an even chance of establishing itself, Kucharski says. That could explain some puzzling aspects of this pandemic, including why the virus did not take off around the world sooner after it emerged in China, and why some very early cases elsewhere-such as one in France in late December 2019, reported on 3 May-apparently failed to ignite a wider outbreak. "Probably about 10% of cases lead to 80% of the spread," Kucharski says. "But we are certainly seeing a lot of concentrated clusters where a small proportion of people are responsible for a large proportion of infections." But in a recent preprint, Adam Kucharski of LSHTM estimated that k for COVID-19 is as low as 0.1. "I don't think this is quite like SARS or MERS, where we observed very large superspreading clusters," Leung says. That seems about right, says Gabriel Leung, a modeler at the University of Hong Kong. They concluded that k for COVID-19 is somewhat higher than for SARS and MERS. 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. In the flu pandemic of 1918, in contrast, the value was about one, indicating that clusters played less of a role.Įstimates of k for SARS-CoV-2 vary. The estimated k for MERS, which emerged in 2012, is about 0.25. In a seminal 2005 Nature paper, Lloyd-Smith and co-authors estimated that SARS-in which superspreading played a major role-had a k of 0.16. The lower k is, the more transmission comes from a small number of people. That's why in addition to R, scientists use a value called the dispersion factor (k), which describes how much a disease clusters. In fact, the latter is the norm, Lloyd-Smith says: "The consistent pattern is that the most common number is zero. But in real life, some people infect many others and others don't spread the disease at all. Without social distancing, this reproduction number (R) is about three. Most of the discussion around the spread of SARS-CoV-2 has concentrated on the average number of new infections caused by each patient. 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. "If you can predict what circumstances are giving rise to these events, the math shows you can really, very quickly curtail the ability of the disease to spread," says Jamie Lloyd-Smith of the University of California, Los Angeles, who has studied the spread of many pathogens. 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 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. Other infectious diseases also spread in clusters, and with close to 5 million reported COVID-19 cases worldwide, some big outbreaks were to be expected. Sometimes a single person infects dozens of people, whereas other clusters unfold across several generations of spread, in multiple venues. Clusters have also occurred aboard ships and at nursing homes, meatpacking plants, ski resorts, churches, restaurants, hospitals, and prisons. A database by Gwenan Knight and colleagues at the London School of Hygiene & Tropical Medicine (LSHTM) lists an outbreak in a dormitory for migrant workers in Singapore linked to almost 800 cases 80 infections tied to live music venues in Osaka, Japan and a cluster of 65 cases resulting from Zumba classes in South Korea. Many similar "superspreading events" have occurred in the COVID-19 pandemic. Centers for Disease Control and Prevention (CDC) that meticulously reconstructed the tragedy. In the following weeks, 53 choir members got sick, three were hospitalized, and two died, according to a 12 May report by the U.S. But one of them had been suffering for 3 days from what felt like a cold-and turned out to be COVID-19. For 2.5 hours the chorists sang, snacked on cookies and oranges, and sang some more. When 61 people met for a choir practice in a church in Mount Vernon, Washington, on 10 March, everything seemed normal. Science' s COVID-19 reporting is supported by the Pulitzer Center.
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