Epidemiological characteristics of a local COVID-19 outbreak caused by Omicron (BA.2.2) variant in Henan province

  Objective  To analyze epidemiological characteristic and transmission process of a local coronavirus disease 2019 (COVID-19) outbreak induced by imported cases from other provinces in Henan province.

  Methods  The data on all local COVID-19 cases in Xiuwu county, Henan province reported during March 18 – 28, 2022 were extracted from National Notifiable Disease Report System. The results of field investigations on the cases were also collected simultaneously. Descriptive statistics was performed on the data collected with SPSS 22.0.

  Results  Totally 45 cases (16 males and 29 females, 14 minor or common patients and 31 asymptomatic infections) were reported during the 11-day outbreak period. The median age (25th percentile P₂₅, 75 percentile P₇₅) of the cases was 23.02 (18.57, 28.64) years. The median (P₂₅, P₇₅) of cycle threshold (Ct) value for Orf1ab gene and N gene for positive cases in primary screening were 23.02 (18.57, 28.64) and 23.10 (18.63, 28.03), respectively. A case clustering involving 18 cases was identified in a local textile factory, with an incidence rate of 6.6%; seven family clusters were also identified, with a family secondary attack rate of 20.78% and a secondary attack rate of 3.79% among close contacts. Of all the cases, 12.5% could attributed the infection to the exposure to the asymptomatic infections being negative for nucleic acid test at that time. The results of genome second generation sequencing for the isolates from 29 infections indicated that the pathogen of the outbreak was severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) variant Omicron (BA.2.2) and the local transmission of the virus variant was traced back to the 2 first generation cases returning home from other epidemic area. The median (P₂₅, P₇₅) of incubation period was 3 (2, 4.25) days and that of generation interval was 3 (2, 3) days. Five generations of transmission were identified for the outbreak . The estimated basic reproductive number (R₀) of this epidemic was 6.14, and the effective reproduction number (Rt) decreased 2 days after the control measures were taken and declined to less than 1 in 8 days after the implementa-tion of control measures.

  Conclusion  The reported COVID-19 outbreak in Henan province was caused by imported cases from other epidemic area and spread mainly in factories, families and schools. The control measures on the outbreak were effective and subsequent transmission was prevented.