Objective To analyze the transmissibility of varicella in Ningbo city from 2020 to 2023 and provide a reference for establishing herd immunity targets for varicella vaccination.

Methods A retrospective analysis of varicella outbreak cases in Ningbo city from 2020 to 2023 that met the research criteria was conducted. Based on multiple outbreaks in Ningbo city, parameters required for the model were provided using data calculation or curve fitting methods. Important epidemic parameters such as attack rate, incubation period (Te), serial interval (Tg), basic reproduction number (R₀), and time-varying reproduction number (R_t) were calculated.

Results From 2020 to 2023, the annual varicella attack rate in Ningbo city showed a wave-like upward trend, with a total of 17 reported public health emergencies of varicella, all of which were school outbreaks. The outbreaks mainly occurred from October to January of the following year, exhibiting seasonality. The exposed population was 43 503, with 414 reported cases, resulting in a total attack rate of 0.952%. The median incubation period of varicella was 12.39 (95%CI: 11.83 - 12.94) days, and the average serial interval was 14.05 (95%CI: 12.75 - 15.35) days. The R₀ value was 3.74 ± 0.73, and the changing trend of R_t indicated that the spread of varicella in Ningbo city basically showed a trend of extinction around the 30th day.

Conclusions The prevention and control measures for varicella outbreaks in Ningbo city are effective, with the overall epidemic trending towards extinction, and the control effect becoming prominent in the third generation of cases. Prevention and control should be strengthened to further establish herd immunity to reduce the peak of the epidemic and shorten its duration.