Seasonal influenza vaccine and frequency of adverse events in health workers receiving two doses of AstraZeneca vaccine: a retrospective cohort study

BACKGROUND: Previous studies suggest that co-administration of COVID-19 and influenza vaccines is safe and maintains immunogenicity. However, evidence on the association between prior seasonal influenza vaccination and adverse events following COVID-19 vaccination remains limited. This study evaluated the relationship between prior influenza vaccination and adverse events after the second dose of the AstraZeneca COVID-19 vaccine among healthcare workers (HWs). METHODS: In this retrospective cohort study, 780 HWs who received two doses of the AstraZeneca vaccine were included. The outcomes were the occurrence and frequency of post-vaccination adverse events. Generalized linear models with binomial distribution and log link function were used to estimate adjusted risk ratios (RRs) for individual adverse events. Poisson regression models estimated adjusted incidence rate ratios (IRRs) for the number of reported adverse events. Relative risk reduction (RRR) and number needed to vaccinate (NNV) were also calculated for selected outcomes. RESULTS: Overall, 95.5% of participants reported at least one adverse event after vaccination, with three adverse events being the most common frequency (n = 210). Prior influenza vaccination was associated with a lower frequency of adverse events (adjusted IRR=0.83, 95% CI: 0.76-0.91). Older age (>40 years) and male sex were also associated with fewer reported adverse events. Influenza vaccination significantly reduced the risks of fever, chills, body pain, gastrointestinal symptoms, and fatigue. Subgroup analyses by age and sex showed generally consistent findings. CONCLUSION: Prior influenza vaccination was associated with fewer adverse events following AstraZeneca vaccination. Older age and male sex were also linked to lower frequencies of reported adverse events.