Safety monitoring of health outcomes following influenza vaccination during the 2023-2024 season among U.S. Commercially-insured individuals aged 6 months through 64 years: Self-controlled case series analyses.

BACKGROUND

Influenza vaccines are reformulated annually to target expected virus strains for the upcoming season. Although the safety of seasonal influenza vaccines is well-established, active safety surveillance of reformulated vaccines is essential to assess occurrence of potential health outcomes among vaccinated individuals. This study evaluated the safety of the 2023-2024 influenza vaccines among U.S. commercially insured individuals aged 6 months to 64 years.

METHODS

We used a self-controlled case series analysis to compare incidence rates of nine safety outcomes following any 2023-2024 seasonal influenza vaccination in pre-specified post-vaccination risk and control intervals among commercially-insured individuals 6 months to 64 years from 3 commercial databases. Outcomes of interest were anaphylaxis, encephalitis/encephalomyelitis/acute disseminated encephalomyelitis, Guillain-Barré syndrome, febrile seizure, transverse myelitis, hemorrhagic stroke, non-hemorrhagic stroke (NHS), transient ischemic attack (TIA), and NHS or TIA. We used conditional Poisson regression to estimate incidence rate ratios (IRRs) and 95 % confidence intervals (CIs), adjusted for outcome-specific seasonality patterns.

RESULTS

We observed nearly 10 million influenza vaccinees across the databases. Combined meta-analysis showed no statistically significant elevation in risk for any outcome except febrile seizure and TIA. A small, statistically significant elevation in risk was observed for febrile seizure (0-1-day risk interval) following standard-dose (IRR: 1.68; [95 % CI: 1.17, 2.43]) or any (IRR: 1.67; [1.16, 2.41]) influenza vaccine. We also observed a statistically significant elevation in risk for TIA (1-21-day risk interval) following a standard-dose (IRR: 1.28; [1.04, 1.59]) or any (IRR: 1.29; [1.05, 1.59]) influenza vaccine.

CONCLUSION

Results from this study show that the risk of adverse health outcomes following the 2023-2024 influenza vaccination was minimal. Continued surveillance is necessary to monitor the safety of annually reformulated seasonal vaccines. The potential risk of febrile seizures and TIA following vaccination must be carefully considered with the known benefits of seasonal influenza vaccination.