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

SARS-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. ### Competing Interest Statement The authors have declared no competing interest. ### Funding Statement The research was funded by the World Health Organization. The funding agency had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. ### Author Declarations I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained. Yes The details of the IRB/oversight body that provided approval or exemption for the research described are given below: All source data were openly available at the initiation of the study. Titles of published studies along with DOIs that were included in the review are provided in Appendix 3. I confirm that all necessary patient/participant consent has been obtained and the appropriate institutional forms have been archived, and that any patient/participant/sample identifiers included were not known to anyone (e.g., hospital staff, patients or participants themselves) outside the research group so cannot be used to identify individuals. Yes I understand that all clinical trials and any other prospective interventional studies must be registered with an ICMJE-approved registry, such as ClinicalTrials.gov. I confirm that any such study reported in the manuscript has been registered and the trial registration ID is provided (note: if posting a prospective study registered retrospectively, please provide a statement in the trial ID field explaining why the study was not registered in advance). Yes I have followed all appropriate research reporting guidelines, such as any relevant EQUATOR Network research reporting checklist(s) and other pertinent material, if applicable. Yes All the data were from publicly available databases. The complete database of extracted information from included studies is provided in Appendix 3.