Clustering and superspreading potential of SARS-CoV-2 infections in Hong Kong

TLDR

Superspreading events, common in SARS and MERS outbreaks, are increasingly recognized as a key driver of SARS‑CoV‑2 transmission, though their exact role remains uncertain. Using contact‑tracing data from 1,038 Hong Kong cases between 23 Jan and 28 Apr 2020, the authors identified and characterized all local infection clusters. The study found 4–7 superspreading events among 51 clusters, with 19 % of cases responsible for 80 % of local transmission; transmission was higher in social settings than households, and quarantining contacts effectively interrupted spread, underscoring the need for rapid contact tracing and social‑setting restrictions.

Abstract

Superspreading events (SSEs) have characterized previous epidemics of severe acute respiratory syndrome coronavirus (SARS-CoV) and Middle East respiratory syndrome coronavirus (MERS-CoV) infections1–6. For SARS-CoV-2, the degree to which SSEs are involved in transmission remains unclear, but there is growing evidence that SSEs might be a typical feature of COVID-197,8. Using contact tracing data from 1,038 SARS-CoV-2 cases confirmed between 23 January and 28 April 2020 in Hong Kong, we identified and characterized all local clusters of infection. We identified 4–7 SSEs across 51 clusters (n = 309 cases) and estimated that 19% (95% confidence interval, 15–24%) of cases seeded 80% of all local transmission. Transmission in social settings was associated with more secondary cases than households when controlling for age (P = 0.002). Decreasing the delay between symptom onset and case confirmation did not result in fewer secondary cases (P = 0.98), although the odds that an individual being quarantined as a contact interrupted transmission was 14.4 (95% CI, 1.9–107.2). Public health authorities should focus on rapidly tracing and quarantining contacts, along with implementing restrictions targeting social settings to reduce the risk of SSEs and suppress SARS-CoV-2 transmission. Cases linked to superspreading events are estimated to account for 80% of all local transmission of SARS-CoV-2 in Hong Kong in a study with implications for public health policies.