RESUMO
Objective: To understand the incidence of delayed vaccination with the national immunization program vaccines among children aged 0-6 years in Xuhui District, Shanghai, and to evaluate the safety of delayed vaccination. Methods: A stratified random sampling was used to obtain six vaccination clinics in Xuhui District, Shanghai. The vaccination records of children 0-6 years from these six vaccination clinics were collected from the Shanghai Immunization Program Information Management System. Adverse events following immunization (AEFI) data were collected from the China Information System for Disease Control and Prevention. Descriptive epidemiology was used to analyze the data. Children were divided into the timely vaccination group and delayed vaccination group according whether they were delayed in vaccination (received one month or more after the recommended age among children aged ≤1 year; received three months or more after the recommended age among children aged >1 year). The safety of four vaccination methods-individual vaccination, simultaneous vaccination, routine vaccination and combined vaccination-were further compared. Differences between groups were compared using chi-square test. Results: From 2019 to 2021, six vaccination clinics in Xuhui District administered 124 031 doses of the national immunization program vaccines among children aged 0-6 years, and delayed vaccinations accounted for 25.99% (32 234/124 031) of these doses. In 2020, the delayed vaccination rate during the first-level COVID-19 public health emergency response period in Shanghai was significantly higher than that in the same period in 2019 (34.70% vs. 24.19%, χ2=136.23, P<0.05). The delayed vaccination rate during the COVID-19 vaccination campaign in 2021 was significantly higher than that in the same period in 2019 (25.27% vs. 22.55%, χ2=82.80, P<0.05). From 2019 to 2021, a total of 475 cases of AEFI were reported in six vaccination clinics, with a reported incidence of 382.97 per 100 000 doses, including 421 cases of common adverse reaction (88.63%, 339.43 per 100 000 doses), 51 cases of rare adverse reaction (10.74%, 41.12 per 100 000 doses) and 3 cases of coincidences (0.63%, 2.42 per 100 000 doses). The reported incidence of AEFI among delayed vaccinations was significantly lower than that among timely vaccinations (291.62 per 100 000 doses vs. 415.05 per 100 000 doses). The incidence of AEFI for the four delayed vaccination methods (individual vaccination, simultaneous vaccination, routine vaccination and combined vaccination) was lower than that for timely vaccination. There were significant differences between the groups except for the routine vaccination group (χ2=9.82, P<0.05; χ2=5.46, P<0.05; χ2=2.97, P>0.05; χ2=11.89, P<0.05). Conclusions: In Xuhui District of Shanghai, 25.99% of doses of the national immunization program vaccines administered to children 0-6 years were delayed. Delayed vaccination does not increase the risk of AEFI compared with timely vaccination.