Effectiveness of DTaP-IPV//PRP-T vaccine against pertussis among infants and toddlers in Zhejiang, China: a retrospective cohort study.

Pertussis poses a significant risk, especially to infants. Although the pentavalent vaccine DTaP-IPV//PRP-T was introduced in China in 2010, real-world data on vaccine effectiveness (VE) in the country are limited. We conducted a retrospective cohort study using data from the Zhejiang Provincial Immunization Information System (ZIIS) and the National Notifiable Disease Surveillance System (NNDSS) to evaluate VE against pertussis between 2017 and 2019. Infants born between 1 January 2017 and 31 May 2019 were included. A multivariate Poisson regression was performed to estimate the effectiveness of different vaccine doses of DTaP-IPV//PRP-T against pertussis. Overall, 1,309,759, 1,215,924, 373,852 and 47,228 participants were enrolled in the 1-, 2-, 3- and 4-dose cohorts, respectively. During follow-up, 185 (14 per 100,000), 72 (6 per 100,000), and 49 (13 per 100,000), and 3 (6 per 100,000) pertussis cases were identified in each cohort, respectively. The adjusted VE of the primary three-dose regimen was 90.8% (95% CI, 79.5%-96.3%) in children aged 4.5-19.5 months, 59.8% (95% CI, 26.5%-80.4%) for the one-dose regimen in children aged 2.5-5.5 months, and 89.0% (95% CI, 69.3%-97.4%) for the two-dose regimen in children aged 3.5-6.5 months. These findings provide robust evidence that the DTaP-IPV//PRP-T vaccine is highly effective against pertussis in infants and toddlers within the Zhejiang setting. Ensuring timely administration and completion of the primary series is paramount for effective pertussis control in China.