Timeliness and dose-dependent protection of pertussis vaccination in children under 5 years: Evidence from a population-based cohort in Zhejiang Province, China.

BACKGROUND

Despite high vaccination coverage, pertussis remains a public health concern in China and worldwide, with limited data on incidence stratified by both age and vaccine dose number.

METHODS

This retrospective cohort study analyzed 2,842,942 children born in Zhejiang between 2015 and 2022, using linked records from the Zhejiang Provincial Immunization Information System and the National Notifiable Disease Surveillance System. Children were followed until pertussis diagnosis, age five, or December 31, 2023. Age-dose-specific and age-vaccination status-specific incidence rates (per 100,000 person-years) were calculated across ten age intervals (0 to <1.5, 1.5 to <2.5, 2.5 to <3.5, 3.5 to <4.5, 4.5 to <5.5, 5.5 to <6.5, 6.5 to <12.5, 12.5 to <19.5, 19.5 to <36, and 36 to <60 months). Poisson regression assessed the pertussis risk by time since fourth dose.

RESULTS

The overall incidence rate was 18.71 per 100,000 person-years under the age of 5 years. Peak incidence (132.87/100,000 person-years) was observed in unvaccinated infants aged 3.5 to <4.5 months. Incidence declined with each additional dose; among children aged 5.5 to <6.5 months, rates decreased from 76.05 (one dose) to 36.87 (two doses) and 11.97 (three doses) per 100,000 person-years. The lowest incidence (5.87/100,000 person-years) was observed in children aged 19.5 to <36 months who received four doses. However, protection waned significantly 3-4 years after the fourth dose (IRR=5.56, 95% CI: 4.33-7.14 vs <1 year), coinciding with a resurgence of incidence at 36 to <60 months of age (15.16 per 100,000 person-years).

CONCLUSION

The findings highlight two vulnerability periods: early infancy (particularly up to 4.5 months of age) before vaccination schedule completion, and preschool years due to waning immunity. These observations support China's 2025 pertussis schedule update, introducing a booster at age six and advancing the initial dose.