Comparative analysis of adverse events following immunization with DTaP vaccine in children before and after immunization schedule adjustment

Abstract

ABSTRACT This study evaluated the impact of immunization schedule adjustment – from a four-dose schedule (3, 4, 5, and 18 months of age) to a five-dose schedule (2, 4, 6, and 18 months of age, and 6y of age) – on the reporting characteristics of adverse events following immunization (AEFIs) with diphtheria, tetanus, and acellular pertussis (DTaP) vaccination in children. Surveillance data in Linping District during 2024 (pre-adjustment) and 2025 (post-adjustment) were collected. Reporting rates, symptom distributions, and dose-specific differences were compared using chi-square tests, rate ratio (RR), and sensitivity analysis. Among 18,869 doses in 2024 (91 AEFIs) and 34,549 doses in 2025 (158 AEFIs), the overall reporting rate of AEFIs did not change significantly (482.27 vs. 457.32 per 100,000 doses; RR = 0.95, 95%CI: 0.83–1.08, P = .442). Local redness and swelling (overall) showed no significant change (RR = 1.04, P = .633), with mild cases (≤2.5 cm) decreasing (RR = 0.20, P = .033) and moderate cases (2.6–5.0 cm) increasing non-significantly (RR = 1.21, P = .138). Local induration (overall) decreased significantly (RR = 0.57, P = .008), driven by moderate induration reduction (2.6–5.0 cm, RR = 0.61, P = .037). The third dose benefited most from the adjustment (RR = 0.40, P < .001). After excluding the fifth dose, reporting rate for doses 1–4 in 2025 was significantly lower than in 2024 (RR = 0.82, P = .006). The fifth dose showed the highest rate (684.84 per 100,000 doses), mostly mild reactions with no serious AEFIs. DTaP schedule adjustment did not increase overall AEFIs rates, while local induration decreased. The primary series (doses 1–4) showed a trend toward improved safety. The newly added fifth dose warrants ongoing monitoring as a key surveillance target.