Active surveillance study on the safety of single versus concomitant administration of Diphtheria, tetanus and acellular pertussis combined vaccine following the immunization schedule adjustment

Objective To compare the incidence of adverse reactions between single administration and concomitant administration of diphtheria, tetanus and acellular pertussis combined vaccine (DTaP) in children aged 2 months to 6 years following the 2025 immunization schedule adjustment.Methods Active surveillance was conducted from January to March 2025 at all vaccination clinics in Shapingba District, Chongqing, among children aged 2 months to 6 years receiving DTaP. Adverse reactions occurring within 30 days after each dose were collected, and the incidence rates of adverse reactions were compared between single administration and concomitant administration groups.Results A total of 4,887 DTaP doses were followed up. The overall incidence rates of adverse reactions were 12.12% (218/1,799) for single administration and 11.59% (358/3,088) for concomitant administration, with no statistically significant difference (χ²=0.30, P=0.583). No significant differences were observed between the two groups in local versus systemic adverse reactions, severity grades (grades 1~4), onset time (≤30 min, 31 min~7 d, 8~30 d), or dose sequence (doses 1~5). For the major adverse reactions including pain, induration, swelling, and fever, no significant differences were found between the two groups. Within the concomitant administration group, the incidence rates of adverse reactions when DTaP was co-administered with one or two other vaccines showed no significant differences compared to single administration.Conclusion Following the 2025 DTaP immunization schedule adjustment, concomitant administration did not increase the risk of adverse reactions in children aged 2 months to 6 years, supporting the continued promotion of concomitant DTaP vaccination strategies.