On 5 December last year — the eve of traditional Christmas gift-giving in Belgium — residents of the Hemelrijck care home near Antwerp were treated to a visit by Sinterklaas, or Santa. But the festive event, intended to spread cheer, turned tragic. Forty staff members and more than 100 residents — at least 26 of whom have since died — were unintentionally infected with the coronavirus SARS-CoV-2 by the costumed volunteer, who also subsequently tested positive.
Superspreading events like this, in which many people are infected at once, typically by a single individual, are a now-familiar feature of the COVID-19 pandemic. Choir practices, funerals, family gatherings and gym classes have all spawned dangerous outbreaks.
Akira Endo, an infectious-diseases modeller at the London School of Hygiene & Tropical Medicine, noticed the telltale signs of superspreading before such events became a staple of COVID-19 news coverage. One clue came from early investigations of cases in which a single person infected up to ten others1. Another curious fact was that outside Wuhan, China, home to the first big outbreak, infected individuals weren’t immediately causing exponential local outbreaks, says Endo, who was one of the earliest to quantify the phenomenon.
This uneven, sputtering form of transmission, in which some individuals infect many people but most infect only a few, if any, is shared by the coronavirus’s cousins — SARS-CoV, which caused the deadly epidemic of severe acute respiratory syndrome (SARS) in 2003, and MERS-CoV, the source of Middle East respiratory syndrome. A similar mode of transmission occurs with the pathogens that cause Ebola, smallpox and tuberculosis.