Safety Monitoring of Health Outcomes following Influenza Vaccination during the 2023-2024 Season among U.S. Medicare Beneficiaries Aged 65 Years and Older
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Background
Influenza vaccination is widely recommended for individuals aged 6 months and older in the United States (U.S.). While the safety of annual influenza vaccines is well established, FDA conducts routine monitoring and evaluation of safety. This study assessed the safety of 2023-2024 influenza vaccines among elderly U.S. Medicare beneficiaries.
Methods
A self-controlled case series (SCCS) analysis compared incidence rate ratios (IRR) of anaphylaxis, encephalitis/encephalomyelitis/acute disseminated encephalomyelitis, Guillain-Barré syndrome (GBS), transverse myelitis, hemorrhagic stroke (HS), non-hemorrhagic stroke (NHS), transient ischemic attack (TIA), and NHS or TIA, following 2023-2024 seasonal influenza vaccinations in risk and control intervals among Medicare beneficiaries aged 65 years and older. We used conditional Poisson regression to estimate IRRs and 95% confidence intervals (CIs) adjusted for event-dependent observation time for certain outcomes, seasonality, and uncertainty from outcome misclassification where feasible. For health outcomes with statistically significant associations, we stratified results by concomitant vaccination status.
Results
We observed a total of 20,258,006 influenza vaccinees among the Medicare population, and no statistically significant elevations of risk for anaphylaxis, encephalitis/encephalomyelitis (with ADEM), GBS, HS, or TM. For the combined NHS/TIA outcome (22-42-day risk window), we observed a small elevation in risk that was statistically significant in both the Fee-For-Service (FFS) and Medicare Advantage (MA) populations that received a high-dose vaccine. This risk was also statistically significant among MA beneficiaries that received any influenza vaccine. Additionally, we observed a small statistically significant risk for the individual TIA outcome (22-42-day risk window) among the MA population that received any influenza vaccine.
Conclusion
Results from this study indicate that the benefits of seasonal influenza vaccines continue to outweigh the risks. The small statistically significant increased risk of stroke outcomes observed in the study must be carefully considered in light of the known benefits of influenza vaccination.
