Impact of the COVID-19 pandemic on the timeliness of pertussis-containing vaccination: A cohort study in Shanghai, China.

The COVID-19 epidemic has significantly disrupted routine childhood immunization programs. This study assesses the impact of different COVID-19 prevalence phases on immunization coverage and timeliness in Shanghai, China. We employed a retrospective cohort design, encompassing children born between September 1, 2018, and August 30, 2023. Children were categorized into three COVID-19 phases based on their expected vaccination dates: baseline period, epidemic period, and pre/post-epidemic period. Kaplan-Meier estimators were utilized to ascertain the cumulative vaccination probabilities, with differences tested using the log-rank method. A multivariate Cox proportional hazards model was conducted to assess the impact of COVID-19 phases on vaccination timeliness. Vaccination timeliness significantly declined when the vaccination window coincided with an epidemic period, particularly for older children and during heightened epidemic activity. The probability of timely vaccination at 2 months was 0.89 (95% CI: 0.86-0.92) in the baseline period and 0.53 (95% CI: 0.51-0.55) in the epidemic period, compared to the pre/post-epidemic period. At 3 months, the probability of timely vaccination decreased 12% (HR = 0.88, 95% CI 0.86-0.90) and 49% (HR = 0.51, 95% CI 0.49-0.53), respectively. Similarly, for vaccinations scheduled at 4 months, the probabilities of timely vaccination were 0.87 (95% CI 0.85-0.89) and 0.49 (95% CI 0.47-0.51) times. At 5 months, the probabilities declined 9% (HR = 0.91, 95% CI 0.87-0.94) and 57% (HR = 0.43, 95% CI 0.40-0.46), correspondingly. Our study assessed the notable decline of vaccination timeliness during epidemic periods, highlighting the need for targeted strategies to mitigate disruptions in routine childhood immunization services.