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ObjectiveTo 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. MethodsDescriptive epidemiological analysis was performed on the cases collected by the AEFI surveillance system. Incidence and coverage of AEFI were then presented. ResultsA 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. ConclusionThe 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.
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Objective: To compare the proportion of children who developed a specified illness in the 7 day post-vaccination window, with the background rate of the same event in the 7 day pre-vaccination window. Study design: Risk interval approach (Self-controlled caseseries). Setting: Well Baby Clinic of Christian Medical College, Vellore. Participants: 1602 healthy infants and under-six children presenting for routine vaccination. Outcome measures: Episode of any illness. Methods: The interviewer enquired about any adverse event or illness experienced by the child for each day of the week preceding the administration of age-appropriate vaccines. A second interview (telephonic) was conducted by the same interviewer one week following vaccine administration to enquire about adverse event(s) experienced by the child for each day of the subsequent week using a similar protocol. Results: With multiple vaccines delivered at appropriate ages, common childhood illnesses that could be reported as adverse events following immunization, except fever (RR=5.7, 95% CI=4.50-7.35), occurred at higher rates pre-vaccination. Risk Ratios of fever following whole cell (RR=9.3, 95% CI=6.43-13.52) and acellular (RR=8.5, 95% CI=3.82-18.91) vaccines were similar, with both showing a decreasing trend with increasing age. The gastrointestinal adverse event profile [diarrhea (RR=0.6, 95% CI=0.14-2.51) and vomiting (RR=1.0, 95% CI=0.14-7.10)] for rotavirus vaccine was similar pre- and post-immunization. Conclusions: Since most adverse events to vaccines are also common childhood illnesses, estimating the background rates of common illnesses is important to accurately ascertain a causal relationship.