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Transmission dynamics of the Delta variant of SARS-CoV-2 infections in South Korea. BACKGROUND: The Delta variant of SARS-CoV-2 is now the predominant variant worldwide. However, its transmission dynamics remain unclear. METHODS: We analyzed 405 local case-patients infected with the Delta variant of SARS-CoV-2 and temporal patterns of viral shedding identified between 22 June and 31 July 2021 in Daejeon, South Korea. RESULTS: Overall, 20% were presymptomatic at the time of epidemiological investigation. We identified six clustered outbreaks, and all were associated with indoor facilities. In 23 household contacts, the secondary attack rate was 63%. We estimated the mean serial interval as 3.26 days (95% credible interval, 2.92-3.60), and 15% (95% confidence interval, 13-18%) of cases seeded 80% of all local transmission. Analysis of the nasopharyngeal swab samples identified virus shedding from the presymptomatic patients, and the highest viral load was observed two days after symptom onset. CONCLUSIONS: Our findings suggest that the Delta variant is highly transmissible in indoor settings and households. Rapid contact tracing, isolation of the asymptomatic contacts, strict adherence to public health measures, and increased uptake of COVID-19 vaccination including booster doses are needed to reduce the community transmission of the Delta variant.