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EuropePMC; 2021.
Preprint in English | EuropePMC | ID: ppcovidwho-294995


Background: The delta variant of SARS-CoV-2 is now the predominant variant worldwide. However, its transmission dynamics remain unclear. Methods We analyzed all case patients in local clusters and temporal patterns of viral shedding using contact tracing data from 405 cases associated with the delta variant of SARS-CoV-2 between 22 June and 31 July 2021 in Daejeon, South Korea. Results Overall, half of the cases were aged under 19 years, and 20% were asymptomatic at the time of epidemiological investigation. We estimated the mean serial interval as 3.26 days (95% credible interval 2.92, 3.60), and 12% of the transmission occurred before symptom onset of the infector. We identified six clustered outbreaks, and all were associated with indoor facilities. In 23 household contacts, the secondary attack rate was 63% (52/82). We estimated that 15% (95% confidence interval, 13–18%) of cases seeded 80% of all local transmission. Analysis of the nasopharyngeal swab samples identified virus shedding from asymptomatic patients, and the highest viral load was observed two days after symptom onset. The temporal pattern of viral shedding did not differ between children and adults ( P  = 0.48). Conclusions Our findings suggest that the delta variant is highly transmissible in indoor settings and households. Rapid contact tracing, isolation of the asymptomatic contacts, and strict adherence to public health measures are needed to mitigate the community transmission of the delta variant.