Background
A SARS-CoV-2 Delta (B.1.617.2) variant outbreak associated with an international airport occurred in Nanjing on 20 July 2021. This study aimed to elucidate the transmission dynamic of the Delta variant, and also evaluate the risk factors for transmission within the household.
Methods
We collected data of COVID-19 cases and close contacts in the outbreak from 20 July 2021 to 24 August 2021, and defined “household contact” as a person living in the same residence of the index case. We estimated the epidemiological parameters, including the basic reproductive number (R0), time-dependent reproductive number (Rt), and the household secondary attack rate (SAR).
Results
A total of 235 cases in 171 households were confirmed. The R0 estimated as 3.73 (95% confidence interval [CI]=2.66-5.15) by exponential growth method. The Rt estimated as 4.36 on July 20, and then decreased to below 1 on August 1 by Bayesian approach. We enrolled 103 households (103 index cases and 234 household contacts) with at least one susceptible household member. The SAR among household contacts was 27.4% (64/234, 95%CI=22.0%-33.4%) throughout 21 days of follow-up, which was higher than SAR among household contacts exposed to the previous strain in 2020 (25.5%, 95%CI=20.1%-31.7%). The household SAR was higher for household contacts of index cases identified during home isolation (42.1%) than during centralized isolation (9.7%) (ORadjusted=6.79, P=0.005). The household SAR among fully vaccinated household contacts (13.6%) was lower than among unvaccinated contacts (32.1%) (ORadjusted=0.34, P=0.011).
Conclusion
Compared to the original strain, Delta variant is more aggressive and transmissible. Effective isolation of index cases from household members could reduce secondary infections. Fully vaccinated household contacts were less susceptible to SARS-CoV-2 infection than unvaccinated contacts.