Secondary attack rate and superspreading events for SARS-CoV-2
Authors: Yang Liu, Rosalind M Eggo, Adam J Kucharski
Journal: The Lancet (2020)

A basic reproduction number, R0, of about 2 was estimated for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) in Wuhan, China, early in the outbreak.1Li Q Guan X Wu P et al.Early transmission dynamics in Wuhan, China, of novel coronavirus-infected pneumonia.N Engl J Med. 2020; (published online Jan 29.)DOI:10.1056/NEJMoa2001316Crossref Scopus (10666) Google Scholar, 2Riou J Althaus CL Pattern of early human-to-human transmission of Wuhan 2019 novel coronavirus (2019-nCoV), December 2019 to January 2020.Euro Surveill. 2020; 252000058Crossref PubMed Scopus (828) Google Scholar However, this value only captures the average dynamics of transmission; a crucial question for control is whether specific situations and settings might be driving the outbreak. The secondary attack rate (SAR), defined as the probability that an infection occurs among susceptible people within a specific group (ie, household or close contacts), can provide an indication of how social interactions relate to transmission risk. Increasingly, outbreak investigations are providing insights into the risk of transmission in different settings. Drawing on data from nine recent reports of secondary transmission associated with a specific event such as a meal or holiday visit (table), we estimated 48 secondary infections occurred among 137 attendees. Assuming that all these secondary infections were generated by a single primary case, which is probable given the short-term nature of the exposure events, would imply a SAR among close contacts of 35% (95% CI 27–44).TableSummary of severe acute respiratory syndrome coronavirus 2 transmission events where the numbers of people exposed and subsequently infected were known, assuming a single primary case, by locationDate of exposureSettingNumber at gathering (excluding primary case)Subsequently infectedHarbin, Heilongjiang, ChinaJan 24, 2020Meal (home)88Shuangyashan, Heilongjiang, ChinaJan 24, 2020Meal (home)134Hangzhou, Zhejiang, ChinaJan 17, 2020Meal (unknown location)11Nanjing, Jiangsu, ChinaJan 23, 2020Meal (unknown location)87Nanjing, Jiangsu, ChinaJan 24, 2020Meal (restaurant)143Enshi, Hubei, ChinaUnknownMeal (unknown location)172Enshi, Hubei, ChinaUnknownMeal (unknown location)4710Haute-Savoie, FranceJan 24–28, 2020Chalet115Hong KongUnknownMeal (home)188Full details are provided in the appendix. Open table in a new tab Full details are provided in the appendix. If transmission is stratified by contacts within and outside of the household, the relationship between R0 and household risk is: R0=SARHNH+ SARCNC, where SARH and SARC are the secondary attack rates within household and wider community (ie, outside household), respectively, and NH and NC are the numbers of at-risk contacts made, respectively.3Yang Y Halloran ME Sugimoto JD Longini IM Detecting human-to-human transmission of avian influenza A (H5N1).Emerg Infect Dis. 2007; 13: 9Crossref Scopus (132) Google Scholar An infection with a high household SAR but a modest R0 would therefore suggest transmission is driven by a relatively small number of high-risk contacts. A large household SAR further suggests that between-household transmission risk is lower; otherwise the observed R0 would be larger. More data are needed to reliably estimate the true within-household and between-household transmission for SARS-CoV-2; recent reports might be biased towards larger transmission events. However, if it transpires that most at-risk contacts have a close relationship with cases, and superspreading events tend to occur at large gatherings of these close contacts, measures to reduce infection risk during such gatherings and subsequent tracing of close contacts of cases might have a disproportionate effect on reducing overall transmission. We declare no competing interests. Download .pdf (.06 MB) Help with pdf files Supplementary appendix