Surveillance on the adverse events following immunization in Dazu District of Chongqing， 2016‒2020

Objective To determine the incidence and characteristics of adverse events following immunization （AEFI） in 2016‒2020 in Dazu District of Chongqing， and evaluate the AEFI surveillance information system and provide scientific evidence for vaccine safety.

Methods Descriptive epidemiological analysis was performed on the cases collected by the AEFI surveillance system. Incidence and coverage of AEFI were then presented.

Results A total of 319 AEFI cases were reported in 2016‒2020. Reported incidence of AEFI was determined to be 24.64/10⁵ doses. The proportion of timely reporting AEFI within 48 hours was 94.04% and that of timely investigation within 48 hours was 98.41%. The proportion of completing the AEFI survey was 100.00% and that of AEFI classification was 99.69%. The male to female ratio was 1.28∶1. Majority of the AEFI cases were less than 1 year old （64.26%， 205/319）， from April to September （68.97%， 220/319）， and reported at 0‒1 day post-immunization （89.34%， 285/319）. Moreover， the reported incidence differed significantly between time periods （χ²=32.481， P<0.05） and time intervals from vaccination to occurrence of AEFI （χ²=961.408， P<0.05）. Coverage of reporting AEFI at county level reached 100％. In addition， non-severe AEFI cases accounted for 98.43% （314/319） and no death case was reported. General events， such as fever， redness， and scleroma， accounted for 80.56% （257/319）， whereas rare adverse events accounted for 15.99% （51/319）. The reported incidence of NIP vaccines was highest in the diphtheria tetanus acellular pertussis combined vaccine， followed by measles and rubella combined attenuated vaccine and diphtheria tetanus combined vaccine. Clinical diagnosis of rare adverse events was mainly allergic rashes.

Conclusion The AEFI surveillance information system in Dazu District of Chongqing achieves efficient monitoring of AEFI. It has proven that the vaccines are generally safe and immunization strategies may be continuously implemented. Furthermore， the sensitivity of AEFI surveillance should be improved through multiple approaches.