Seasonal Influenza Vaccine and Frequency of adverse events in health workers receiving two doses of AstraZeneca vaccine: a retrospective cohort study

Objectives Limited published study was found regarding the number of adverse reactions of the Covid-19 vaccine in relation with receiving seasonal influenza vaccine. The purpose of this study was to determine the relationship between receiving the seasonal influenza vaccine and the frequency of side effects of the AstraZeneca vaccine. Methods This retrospective cohort study was conducted on 780 Healthcare Workers (HWs). The interest outcome was the adverse reactions to the AstraZeneca vaccination. Generalized linear model was done to determine the relationship. Relative risk reduction (RRR) and number needed to vaccinate (NNV) indexes were also estimated. Results Majority (95.5%) of the HWs reported at least one side effect of AstraZeneca vaccine. The incidence risk ratio shows that the incidence of side effects decreased by 23% in HWs aged > 40 (compared to aged ≤ 40), 29% in male (compared to female), and 29% in the subjects who received influenza vaccine compared to people who have not received influenza vaccine. The estimation of RRR varied between 11%-69%. The NNV of influenza vaccine was 5–58 subjects. Conclusions Our findings quantitatively showed a range of risk of side effects caused by the AstraZeneca vaccine in relation to the seasonal influenza vaccine. In general, the high influenza vaccine coverage, older age, and male had a relatively protective effect against the number of side effects of the AstraZeneca vaccine.