Incremental effectiveness of the second dose of mumps-containing vaccine among children 9 months to 6 years in Hefei: A matched case-control study

To evaluate the incremental vaccine effectiveness (IVE) of the second dose of mumps-containing vaccine (2^thMuCV) compared to first dose and to provide evidence for optimizing mumps immunization strategies. We conducted a 1:1 matched case-control study of mumps cases reported in Hefei from 2018 to 2024 (aged 9 months to 6 years), selecting community controls without a history of mumps infection and matching them by age and sex. A total of 3483 pairs of cases and controls were included, of whom 21.76% and 23.02% had received two-doses MuCV respectively. The overall IVE of 2^thMuCV was 22.35% (95%CI: 3.08%-37.78%) beyond that provided by the first dose alone. Subgroup analyses revealed a higher vaccine effectiveness (IVE) in children with a vaccination interval of <1 year (32.67%; 95% CI: 8.44 to 50.49) compared to those with an interval of ≥1year (8.97%; 95% CI: -25.54 to 34.00). VEs for the group <3 and ≥3 years old were 60.00% (95% CI: 16.72%-80.79%) and 16.88% (95% CI: -5.08%-34.25%), respectively. The IVEs waned over time, within and beyond three years after receipt of the 2^th MuCV vaccine dose were 43.02% (95% CI: 19.08%-59.88%) and 34.37% (95% CI: -13.79%-62.15%). 2^thMuCV provides additional protection against mumps, although diminished over time. The vaccine schedule with two-dose MuCV should be implemented among children in Hefei. The optimal age for the second dose needs to be further evaluated. Additionally, it is important to explore the potential benefits of providing a booster dose prior to enrollment in preschool or childcare settings.