How were DTP-related adverse events reduced after the introduction of an acellular pertussis vaccine in Chile?

Chile has a passive surveillance system of adverse events following immunization (AEFI) that allows monitoring and evaluating the safety profile of the vaccines administered. Between 2018 and 2019, the National Immunization Program (NIP) changed from a pentavalent whole-cell pertussis vaccine (wP) to a hexavalent (DTaP-IPV-HepB-Hib) acellular pertussis vaccine (aP) for children <2 years. Objectives: To describe the trend in the frequency of adverse events (AE) records associated to pertussis component vaccines between January 1^st, 2015 and June 30^th, 2020 in infants younger than 2-years-old in Chile, by reviewing the records submitted to the AEFI NIP, stratified by DTP-vaccine type, wP or aP. Materials and methods: This was a retrospective observational study including all AEFI records of DTP (either aP or wP)-containing vaccines in the described sample. A descriptive analysis was performed according to vaccine type and AEFI, using MedDRA terminology. Results: The total number of AEFI reports was 1,697: 815 corresponding to wP vaccines, 417 to aP vaccines, and 465 with unknown type. The reporting rates for the years 2015 to 2020 were 40.1, 56.2, 37.1, 24.7, 19.1, and 12.2 per 100,000 doses administered, respectively. The most reported AEFI were injection site erythema (42.9%), pyrexia (35.7%), and pain at the injection site (29.2%). Among all cases, 5.8% were SAEs (n = 98), 5.9% were SAEs for wP vaccines (n = 48) and 5.3% were for aP vaccines (n = 22). Discussion: A significant decrease in AEFI reports was observed as of 2018, the year that the DTaP-IPV-HepB-Hib was introduced in the NIP.