Estimating diphtheria, acellular pertussis, tetanus vaccine (DTaP) effectiveness in preventing pertussis in children aged three months to six years old in Shenzhen area, China, case control study.

BACKGROUND

Pertussis is a highly contagious respiratory disease caused by Bordetella pertussis. In recent years, it has resurged in many parts of the world. Surveillance data from Longgang District, Shenzhen (2017-2023) show that the age distribution of cases has shifted from being dominated by infants aged 3-11 months (100 % in 2017) to children aged three to six years (>50 % in 2023), suggesting that waning vaccine-induced immunity may be a key driver of the resurgence. This study aimed to estimate the real-world effectiveness of DTaP among children aged three months to six years to inform evidence-based optimisation of immunization strategies.

METHODS

We employed a test-negative design within a 1:2 matched case-control framework. Cases were defined as all laboratory-confirmed pertussis patients aged three months to six years reported by healthcare facilities in Longgang District, Shenzhen, between 2013 and 2023. Controls were children who tested negative for pertussis and attended the same healthcare facility as their matched case, were of the same sex and exact age (in months), and were selected using a random-number-based matching procedure. The required sample size (140 cases and 280 controls) was calculated a priori, assuming an expected vaccine effectiveness of 85 %, 90 % power, and a two-sided α of 0.05. Data collection continued until the target sample size was reached. Conditional logistic regression was used to estimate the effectiveness of the DTaP vaccine.

RESULTS

A total of 219 cases and 438 controls were included. The overall VE of DTaP in preventing pertussis in children aged three months to six years was 72.60 % (95 % CI:60.12 %,81.18 %). The VE for one to two doses, three doses, and four doses of DTaP was 65.76 % (95 % CI:45.89 %,78.54 %), 59.16 % (95 %CI,29.45 %,76.72 %), and 79.77 % (95 %CI,68.75 %,87.05 %), respectively. Analysis of the four-dose vaccinated cohort suggested a potential decline in VE over time; however, this trend should be interpreted with caution as confidence intervals overlapped substantially across time strata. Nevertheless, the protective effect remained at 76.10 % even 36-47 months after completing vaccination.

CONCLUSIONS

VE increased with the number of doses administered. Although a gradual decline in VE over time was suggested after completing the primary series, conclusive evidence for waning immunity was limited by the precision of our estimates, which indicated a nonlinear trend. Our findings underscore the importance of ensuring universal completion of the four-dose DTaP series, which provided robust protection for at least the first four years of life.