Stroke After Influenza Vaccines in Older Adults in the US, 2016 to 2019

This case series investigates whether an increase in stroke risk occurred after influenza vaccination among fee-for-service Medicare beneficiaries during the influenza seasons from 2016 to 2019.


Introduction
The US Food and Drug Administration (FDA) investigated stroke risk following COVID-19 bivalent and influenza vaccines among Medicare beneficiaries 65 years or older in 2022 and 2023. 1 The selfcontrolled case series (SCCS) study found no consistent evidence of stroke risk in the days following COVID-19 bivalent vaccination.However, an association between stroke and influenza vaccination was detected.To determine whether stroke risk increased following influenza vaccination, we investigated the 2016 to 2017, 2017 to 2018, and 2018 to 2019 influenza seasons.

Methods
2][3] Fixed risk windows (1-21 and 22-42 days) were compared with a control window (43-90 days).For each influenza season, the study period began on the first Sunday of August and ended 1 day before the start of the subsequent season.Participants did not reside in a nursing home and were enrolled in fee-for-service Medicare for at least 1 year before vaccination.We estimated incidence rate ratios (IRR) for each season using conditional Poisson

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Open Access.This is an open access article distributed under the terms of the CC-BY License.This study was classified as public health surveillance and was exempted from institutional review board approval and informed consent under the Common Rule.We followed the Appropriate Use and Reporting of Uncontrolled Case Series in the Medical Literature reporting guideline.

Results
The

Discussion
Although we detected associations in the primary and age subgroup analyses, we did not observe consistent increased stroke risk following high-dose or adjuvanted influenza vaccination from 2016 to 2019.While the multiple tests conducted may have increased type I error rate, we designed our system to be sensitive and looked for consistency when interpreting results.The associations we identified were not consistent across outcomes, risk windows, age subgroups, and influenza seasons.
Additionally, clusters observed in the temporal scan were not consistent with the associations identified through the SCCS framework.
Our study has some limitations.Mainly, we included only vaccinated beneficiaries.While influenza infection is a known trigger for stroke, we could not account for any protective effect of vaccination.The clinical significance of any potential risk of stroke following vaccination must be carefully considered with known benefits of influenza vaccination. 4,5 calculated the risk difference (RD) per 100 000 doses.A retrospective temporal scan identified clusters of increased stroke risk within 90 days following vaccination.Data were analyzed from August 7, 2016, to August 3, 2019.Time-fixed confounders such as race and ethnicity are implicitly controlled by the design.