Abstract
ObjectivesSuper-spreading events caused by overdispersed secondary transmission are crucial in the transmission of COVID-19. However, the exact level of overdispersion, demographics, and other factors associated with secondary transmission remain elusive. In this study, we aimed to elucidate the frequency and patterns of secondary transmission of SARS-CoV-2 in Japan. MethodsWe analyzed 16,471 cases between January 2020 and August 2020. We generated the number of secondary cases distribution and estimated the dispersion parameter (k) by fitting the negative binomial distribution in each phase. The frequencies of the secondary transmission were compared by demographic and clinical characteristics, calculating the odds ratio using logistic regression models. ResultsWe observed that 76.7% of the primary cases did not generate secondary cases with an estimated dispersion parameter k of 0.23. The demographic patterns of primary-secondary cases differed between phases, with 20–69 years being the predominant age group. There were higher proportions of secondary transmissions among older individuals, symptomatic patients, and patients with 2 days or more between onset and confirmation. ConclusionsThe study showed the estimation of the frequency of secondary transmission of SARS-CoV-2 and the characteristics of people who generated the secondary transmission.
Highlights
Epidemiological findings on coronavirus disease 2019 (COVID-19) have accumulated at an unprecedented rate
A key finding is the heterogeneity in the number of secondary transmissions, which is often characterized as overdispersion
The remaining 16,471 cases were included as primary cases for the principal analysis of secondary transmission
Summary
Epidemiological findings on coronavirus disease 2019 (COVID-19) have accumulated at an unprecedented rate. A key finding is the heterogeneity in the number of secondary transmissions, which is often characterized as overdispersion. A study in Hong Kong revealed that only 19% of infected persons with COVID-19 generated 80% of all transmissions. A study conducted in Japan at the beginning of the COVID-19 outbreak examining 110 cases, including 11 clusters, identified that the majority of infected individuals did not generate any secondary transmission (Nishiura et al, 2020b). The characteristics of clusters at the early stage of the outbreak were analysed in Japan(Furuse et al, 2020b). Based on these findings, Japan developed a cluster-based approach to COVID-19 management that focuses on identifying and preventing clusters (super-spreading events) to suppress transmission (Oshitani, 2020)
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