Objective  To analyze and compare prevalence characteristics of coronavirus disease 2019 (COVID-19) cluster epidemic-related secondary infections caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) Delta and Omicron variant in Ningbo city, and to provide a basis for optimizing and adjusting measures on COVID-19 prevention and control.

  Methods  From the National Information System for Disease Control and Prevention and the reports of field surveys, we extracted the information on 171 COVID-19 cases with SARS-CoV-2 Delta and Omicron variant infection identified among close contacts of 37 local COVID-19 outbreaks in Ningbo city from December 1, 2021 to May 31, 2022. Descriptive analysis was performed to analyze prevalence characteristics of the cases and the incubation period of secondary infection was calculated for the cases with definite exposure time to index cases.

  Results  Of all the cases, 80.70% (n = 138) and 19.30% (n = 33) were confirmed patients and asymptomatic infections; 64.33% (n = 110) and 35.67% (n = 61) were infected with SARS-CoV-2 Delta and Omicron variant, respectively. The proportion of asymptomatic infection was significantly higher in the cases infected with Omicron variant than that in the cases with Delta variant (54.10% vs. 0, P < 0.05). The mean incubation period was 3.45 ± 0.44 days for the 22 confirmed cases (15 and 7 with Delta and Omicron variant infection) after a definite exposure to the index case. The longest incubation period of Delta and Omicron variant infection were 8 and 6 days. The mean incubation period was 3.60 ± 0.58 and 3.14 ± 0.63 days for Delta and Omicron variant infection, with no significant difference (P > 0.05). The interval time between the last exposure and the first positive detection of nucleic acid was ≤ 7 days for the cases with Omicron variant infection. The Ct value of N gene at the first positive nucleic acid detection was significantly higher in the patients with Omicron variant infection than that in those with Delta variant infection (P < 0.05), but the average hospitalization days of the cases with Omicron variant infection was significantly lower than that of cases infected with Delta variant (P < 0.05).

  Conclusion  The incubation period of SARS-CoV-2 Omicron variant infection is less than 7 days and Omicron variant infection is less pathogenic than Delta variant infection. The results suggest that the recommended managent and control measures for close contacts are 7 days′ centralized isolation and 3 days′ home-based health monitoring.