Evaluation of the real-world protective effect of mumps-containing vaccines among children under 15 years old in Taizhou: A 1:2 matched case-control study

Background This study aimed to evaluate the real-world protective effectiveness of the mumps-containing vaccine (MuCV) among children under 15 years of age in Taizhou City, and to analyze the impact of various vaccination-related factors on vaccine effectiveness (VE), so as to provide localized epidemiological evidence for optimizing mumps immunization strategies in children. Methods A 1:2 matched case-control study was conducted, enrolling 1,405 clinically or laboratory-confirmed mumps cases aged under 15 years in Taizhou City from 2021 to 2025, with 2,810 controls matched by sex, stage of schooling, area of residence, and date of birth. Conditional logistic regression models were used to analyze factors influencing VE. To address sparse data bias, Firth penalized logistic regression sensitivity analyses were additionally performed using R software. Results The proportion of unvaccinated individuals was significantly higher in the case group (5.3%) than in the control group (0.5%). Receiving at least one dose of MuCV provided a VE of 75.7% (95% CI : 64.8% ~ 83.2%). VE showed an increasing trend with age: no significant protective effect was observed in infants and young children (aged 0 ~ 2 years); for kindergarten children (aged 3 ~ 5 years), VE was 55.2% (95% CI : 22.6% ~ 74.1%); for primary school children (aged 6 ~ 11 years), VE was 86.7% (95% CI: 72.8% ~ 93.5%); and for secondary school children (aged ≥12 years), VE was 92.9% (95% CI : 76.4% ~ 97.9%). However, sparse data bias in the younger age subgroups warrants cautious interpretation. During the single-dose immunization schedule period, VE was 80.4% (95% CI : 70.3% ~ 87.0%), whereas during the two-dose period, no statistically significant protective effect was observed. This result should be interpreted with caution, as the extremely small number of unvaccinated children precluded a reliable assessment of VE, and should not be interpreted as evidence that the two-dose schedule is ineffective. Using unvaccinated individuals as the reference, the P values for the < 18 months and ≥18 months groups were 0.456 and 0.460, respectively, with no statistical significance. Because only 14 unvaccinated controls were available, the OR and VE could not be reliably estimated. The interval between two doses (≤10 months vs. > 10 months) and time since last dose (≤5 years vs. > 5 years) did not show a significant effect on VE (P > 0.05). Due to the limited sample size, these results should be interpreted with caution. Sensitivity analyses and Firth penalized logistic regression analyses yielded results consistent with the main findings, collectively supporting the robustness of our conclusions. Conclusion MuCV provided significant overall protection against mumps in children under 15 years of age. Subgroup analyses were limited by the extremely high vaccination coverage and the consequent scarcity of unvaccinated individuals. Strengthening vaccination management for infants, young children, and kindergarten children is recommended. Future studies with larger sample sizes and extended follow-up are needed to clarify the incremental value of the two-dose schedule and the pattern of waning immunity.