SuperspreadingEvents

Superspreading events (SSEs) are incidents in which one or a few index cases infect a disproportionately large number of people in a particular place and time, fueling rapid clusters within an outbreak. They reflect transmission heterogeneity, not just high average contagiousness.

They arise from the interaction of biological, behavioral, and environmental factors: high viral load and shedding, prolonged exposure, close contact, indoor settings with poor ventilation, loud speaking or singing, and large crowds. The dispersion of transmission is often summarized by the dispersion parameter k; small k values indicate that a minority of cases account for most transmissions, enabling SSEs.

Notable SSEs have shaped several outbreaks. During the SARS-CoV outbreak of 2003, hospital, hotel, and institutional clusters were linked to a few super-spreading events. In the COVID-19 pandemic, numerous SSEs occurred at conferences, religious gatherings, choirs, and meatpacking plants, illustrating how indoor crowding and activities that generate aerosols can drive rapid spread.

Public health responses aim to reduce SSE risk by improving ventilation, limiting indoor crowding, masking in high-risk settings, vaccination, rapid testing, and prompt isolation, along with targeted contact tracing to identify exposure networks from large events. Understanding SSEs helps explain why some outbreaks accelerate suddenly and why interventions focused on high-risk settings can be especially effective.