Transmission characteristics of a COVID-19 outbreak caused by Omicron variant BA.5.2 in Yiwu city, China

  Objective  To analyze transmission characteristics of a coronavirus disease 2019 (COVID-19) outbreak caused by Omicron variant BA.5.2 in Yiwu city of Zhejiang province, China.

  Methods  Medical records and the information on epidemiological investigation, laboratory tests were collected for all local COVID-19 cases diagnosed during August 2 – 21, 2022 – a period of an COVID-19 outbreak in Yiwu city. The transmission characteristics of the outbreak was analyzed using indicators including family secondary attack rate, transmission generations (TG), serial interval (SI), the time interval from symptom onset to positive nucleic acid test, as well as time-varying reproduction number (R_t).

  Results  The indicator case of the outbreak was confirmed with the history of living in high-risk area of other provinces and close contact with COVID-19 infected persons 7 days before the onset of disease symptoms. Genome sequencing of the virus isolated from the indicator case's secondary cases indicated that the viral strain of the outbreak was severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) Omicron variant BA.5.2. Totally 713 cases (343 males and 370 females) from 13 subdistricts (except for Chi'an township) of Yiwu city were confirmed during the outbreak lasting for 20 days. The median age of the cases (percentile 25 P₂₅, percentile 75P₇₅) was 33 (20, 74) years. A total of 138 household clusters (involving 74.2% of all infections) were identified in the outbreak. For the 101 household clusters with clear transmission chain, the secondary attack rate was 62.2% (95% confidence interval: 56.5% – 67.5%); 39.6% of the household clusters affected all family members of the households involved; the median TG (P₂₅, P₇₅) was 1.7 (1.0, 2.8) days for the primary cases and 197 secondary cases and the TG was equal or less than 2 days for 50.8% of the cases being involved. For 65 household clusters, the median SI (P₂₅, P₇₅) was 2.0 （1.0, 3.0）days for the primary cases and 99 secondary cases with clinic symptoms and the SI was equal or less than 2 days for 44.4% of the cases being involved. Of the 364 cases (51.1% of the cases undergoing epidemiological surveys) with self-reported symptoms, 64.3%, 23.4%, and 20.1% reported fever, cough, and sore/dry throat, respectively. For the 364 symptomatic cases, the median time interval (P₂₅, P₇₅) from symptom onset to positive nucleic acid test was 0（0, 1）day and the time interval was one day for 67.7% of the cases and two days for 22.6% of the cases. No significant differences were observed in gender, age, site and daily time point of specimen sampling for nucleic acid test, and cycle threshold (Ct) value between the cases with and without lag time (day) of positive nucleic acid test from symptom onset. The maximum R_t was 6.18 in early period of the COVID-19 outbreak and the R_t decreased to less than one week after the implementation of containment measures.

  Conclusion  The COVID-19 outbreak caused by Omicron variant BA.5.2 was characterized by high family secondary attack rate, short transmission generation and serial interval, and a high proportion of cases with lag time of positive nucleic acid test from symptom onset.