Assessment of potential adverse events following the 2022–2023 seasonal influenza vaccines among U.S. adults aged 65 years and older

Background While safety of influenza vaccines is well-established, some studies have suggested potential associations between influenza vaccines and certain adverse events (AEs). This study examined the safety of the 2022–2023 influenza vaccines among U.S. adults ≥ 65 years. Methods A self-controlled case series compared incidence rates of anaphylaxis, encephalitis/encephalomyelitis, Guillain-Barré Syndrome (GBS), and transverse myelitis following 2022–2023 seasonal influenza vaccinations (i.e., any, high-dose or adjuvanted) in risk and control intervals among Medicare beneficiaries ≥ 65 years. We used conditional Poisson regression to estimate incidence rate ratios (IRRs) and 95 % confidence intervals (CIs) adjusted for event-dependent observation time and seasonality. Analyses also accounted for uncertainty from outcome misclassification where feasible. For AEs with any statistically significant associations, we stratified results by concomitant vaccination status. Results Among 12.7 million vaccine recipients, we observed 76 anaphylaxis, 276 encephalitis/encephalomyelitis, 134 GBS and 75 transverse myelitis cases. Only rates of anaphylaxis were elevated in risk compared to control intervals. With all adjustments, an elevated, but non-statistically significant, anaphylaxis rate was observed following any (IRR: 2.40, 95% CI: 0.96–6.03), high-dose (IRR: 2.31, 95% CI: 0.67–7.91), and adjuvanted (IRR: 3.28, 95% CI: 0.71–15.08) influenza vaccination; anaphylaxis IRRs were 2.54 (95% CI: 0.49–13.05) and 1.64 (95% CI: 0.38–7.05) for persons with and without concomitant vaccination, respectively. Conclusions Rates of encephalitis/encephalomyelitis, GBS, or transverse myelitis were not elevated following 2022–2023 seasonal influenza vaccinations among U.S. adults ≥ 65 years. There was an increased rate of anaphylaxis following influenza vaccination that may have been influenced by concomitant vaccination.