To investigate the epidemiological characteristics of mumps breakthrough cases and evaluate vaccine effectiveness among children born in Shapingba district, Chongqing from 2008 to 2018, thus providing evidence for optimizing mumps immunization strategies and formulating precise prevention and control measures.

A retrospective cohort study was conducted among children born between January 1, 2008 and December 31, 2018 and residing in Shapingba district. Basic information and vaccination records for mumps-containing vaccine (MuCV) were obtained from the Chongqing Immunization Planning Information Management System. Mumps incidence data were retrieved from the China Information System for Disease Control and Prevention by matching child′s name, date of birth, gender, and current address. Participants were categorized into 1-dose, 2-dose, and 3-dose groups according to MuCV vaccination doses. Follow-up started from the completion date of each dose and ended at either mumps diagnosis or the study cutoff date (December 31, 2023). Person-years were calculated through the exact calculation method to determine incidence density. Univariate Poisson regression was employed to estimate disease risk across groups, while analyzing demographic characteristics, onset time, and time intervals from MuCV vaccination to mumps onset among breakthrough cases.

The total incidence density of mumps breakthrough cases among children born in Shapingba district, Chongqing from 2008 to 2018 was 21.16 per 10 000 person-years (1 149 cases). The overall incidence of breakthrough cases showed a bimodal distribution pattern, with peaks occurring from April to June and in September. The median intervals from MuCV vaccination to disease onset for breakthrough cases in the 1-dose, 2-dose, and 3-dose groups were 36.50 (24.02, 51.09) months, 59.60 (45.37, 74.00) months, and 80.40 (62.22, 97.49) months, respectively, with differences among the groups (H = 89.63, P < 0.001). The total incidence density of breakthrough cases among children who received 1, 2, and 3 doses of MuCV was 24.82, 16.60, and 10.05 per 10 000 person-years, respectively. Children receiving 2-dose MuCV had a 33% reduced risk of mumps breakthrough infection compared to those receiving 1-dose MuCV (VE = 33%, 95%CI: 24%–41%, P < 0.001).

The total incidence density of mumps breakthrough cases was relatively high among children born in Shapingba district, Chongqing from 2008 to 2018, with peak incidence occurring from April to June and in September. Two-dose MuCV vaccination can extend the duration of immune protection and significantly reduce the risk of breakthrough infection.