Community transmission of COVID-19 outbreaks caused by SARS-CoV-2 Delta variant in 3 areas of Zhejiang, December 2021

  Objective  To analyze epidemiological characteristics of COVID-19 outbreaks caused by SARS-CoV-2 Delta variant in 3 areas of Zhejiang province in December 2021.

  Methods  The case data and epidemiological investigation reports were collected from China's Infectious Disease Information System and local CDCs. The epidemiological characteristics of the cases were described and incubation period, serial interval, and time-varying reproduction numbers (R_t) were estimated.

  Results  A total of 490 indigenous COVID-19 cases were reported in Ningbo, Shaoxing and Hangzhou of Zhejiang from 4^th December to 27^th December 2021. All the cases were laboratory confirmed. In Ningbo, all the 74 cases lived in Jiaochuan sub-district of Zhenhai district. The incidence peak period was during 8^th−13^th December. The epidemic duration and community transmission duration was 13 days and 6 days. In Shaoxing, 387 cases were reported in Shangyu district (99.22%) and Yuecheng district (0.78%). The incidence peak period was during 12^th−16^th December. The epidemic duration and community transmission duration was 21 days and 13 days. In Hangzhou, 29 cases were reported in five districts. The incidence peak occurred on 9^th December. The epidemic duration and community transmission duration was 13 days and 4 days. Among all the cases, the male to female ratio was 0.88∶1. The age of the cases ranged from two months to 98 years old, and average age was 48.4 years. The cases were mainly workers (162 cases) and farmers (113 cases). The clinical classifications were mainly mild (194 cases, 39.59%) and common (291 cases, 59.39%). Transmissions mainly occurred in families and closed spaces. Close contacts or secondary contacts investigation and community screening were the main ways to find cases. The median of incubation period and serial interval was 2.53 (P₂₅-P₇₅: 1.88–3.32) d and 2.00 (P₂₅–P₇₅: 1.21–3.09) d. R_t decreased below 1 on 10^th, 12^th and 11^th day in Ningbo, Shaoxing and Hangzhou respectively.

  Conclusion  The COVID-19 outbreaks in 3 areas of Zhejiang spread rapidly. Closed spaces such as home, markets and crowd gathering activity places had high-risk of transmission. Scientific and accurate investigation and effective management of people at high risk for exposure would facilitate the rapid control of the epidemic.