Superspreading of SARS-CoV-2: a systematic review and meta-analysis of event attack rates and individual transmission patterns

Abstract

SUMMARYSARS-CoV-2 superspreading occurs when transmission is highly efficient and/or an individual infects many others, contributing to rapid spread. To better quantify heterogeneity in SARS-CoV-2 transmission, particularly superspreading, we performed a systematic review of transmission events with data on secondary attack rates or contact tracing of individual index cases published before September 2021, prior to emergence of variants of concern and widespread vaccination. We reviewed 592 distinct events and 9,883 index cases from 491 papers. Meta-analysis of secondary attack rates identified substantial heterogeneity across 12 event types/settings, with the highest transmission (25–35%) in co-living situations including households, nursing homes, and other congregate housing. Among index cases, 67% produced zero secondary cases and only 3% (287) infected >5 secondary cases (“superspreaders”). The highest percentage of superspreaders was among symptomatic individuals, individuals aged 49–64 years, and individuals with over 100 total contacts. However, only 55% of index cases reported age, sex, symptoms, real-time PCR cycle threshold values, or total contacts. Despite the limitations, our review highlighted that SARS-CoV-2 superspreading is more likely in settings with prolonged close contact and among symptomatic adults with many contacts. Enhanced reporting on transmission events and contact tracing could help explain heterogeneity and facilitate control efforts.