Overdispersion of SARS-CoV-2 transmission in Japan: Demographic characteristics and temporal change

概要

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学
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位 論 文 要 約
A b s t r a c t ）

博士論文題目 Title of dissertation
Overdispersion of SARS-CoV-2 transmission in Japan: Demographic characteristics and temporal change
（日本における SARS-CoV-2 の感染伝播の過分散について）

東北大学大学院医学系研究科
病理病態学講座

医科学専攻
微生物学分野

氏名 Name
髙 勇羅
Heterogeneity in transmission of infectious diseases has been extensively studied since the emergence of severe acute
respiratory syndrome (SARS). Now, it is known that superspreading events (SSEs) are essential factors that contribute to
sustained transmission and also play an important role in the control of various infectious diseases. The overdispersion of
transmission has also been reported in severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) in many
countries since the early stage of the pandemic. SSEs caused by overdispersed secondary transmission are crucial in the
transmission of coronavirus disease 2019 (COVID-19). Based on this characteristic, Japan has been implementing
cluster-based approaches that focus on identifying and preventing SSEs to minimize the impact of COVID-19 since the
beginning of the pandemic. However, the demographics and other factors associated with secondary transmission of
severe acute respiratory coronavirus 2 (SARS-CoV-2) remain elusive in Japan. Furthermore, studies comparing
transmission heterogeneity among new variants of concerns (VOCs), especially for Omicron, are limited. Thus, this
dissertation aimed to elucidate the patterns of secondary transmission of SARS-CoV-2 in Japan by using available open
data. In addition, the study clarified how the transmission heterogeneity changes with the emergence of VOCs (Alpha,
Delta, and Omicron) by using detailed contact tracing data in Yamagata, Japan.
I counted the number of secondary cases of each identified case based on the transmission pairs for the first two waves in
Japan. The demographic patterns of the primary and secondary cases differed between the epidemic periods, with 20–69
years being the predominant age group regarding primary cases. There were higher proportions of secondary cases
among people aged 60–69 years (28.6%), symptomatic cases (24.8%), and those with 2 days or more between onset and
confirmation (27.4%). Thereafter, I estimated the time-varying dispersion parameter (kt) by fitting a negative binomial
distribution separately by transmission generation at each point in cases from Yamagata between November 1, 2020 and
January 19, 2022. Our results showed that even after the emergence of new variants, there was transmission
overdispersion of SARS-CoV-2, with changes in the degree of overdispersion during the progression of each wave.
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Further interventions should be performed based on these characteristics. Moreover, I believe in the importance of
continuously measuring transmission dynamics in each region and taking appropriate countermeasures. Establishing a
feasible system to obtain epidemiological parameters is vital to making this possible, and a continuous evaluation of its
cost-effectiveness, depending on the epidemic situation, is also warranted for public health implications. ...