Researchers addressed the association between older individuals who received an influenza vaccine and their risk of stroke following vaccination.
After gathering data from over 14 million Medicare beneficiaries 65 and older, researchers found no significant association between influenza vaccinations and an increased risk of stroke. Despite the fact there were some associations between vaccination and stroke identified, they were not consistent enough for researchers to suggest that patients at risk of stroke should not receive their annual influenza vaccine.
“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. The self-controlled 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,” wrote authors of a study published in JAMA Network Open.1
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Researchers went back to the pre-pandemic flu seasons of 2016 to 2019 in order to reach a greater understanding of the influenza vaccine’s effects on cardiovascular health and overall stroke risk. Previous studies have explored the vaccine’s impact on cardiovascular disease (CVD) as a whole, with researchers finding a decrease in major cardiovascular events, specifically myocardial infarction and cardiovascular death.2
“Differing viewpoints exist regarding the impact of influenza vaccination on CVDs. While certain observational investigations suggest a favorable correlation between influenza vaccination and the reduction in occurrences of cardiovascular incidents like acute myocardial infarction (MI), contrasting epidemiological studies propose the limited efficacy of influenza vaccines,” wrote the authors of a study published in Scientific Reports.
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In Lu et al’s article, however, researchers put a deeper focus on the incidence of stroke in Medicare beneficiaries who have received a flu vaccination. The authors explored various types of stroke, including nonhemorrhagic stroke (NHS), transient ischemic attack (TIA), NHS and/or TIA (NHS/TIA), and hemorrhagic stroke (HS). All participants were 65 or older and were separated into 1 of 3 age subgroups: 65 to 74, 75 to 84, and 85 years or older.1
Regarding the starting and stopping points for each flu season included in the study, the period—or that respective year’s “flu season”—began on the first Sunday of August and ended 1 day before the start of the subsequent season. Researchers then estimated incidence rate ratios (IRRs) to calculate the risk of stroke for all 3 seasons and age subgroups. Finally, patients were separated into fixed risk windows of 1-21 or 22-42 days, with the control window lasting from days 43-90.
This SCCS study included a total of 14,669,716 Medicare beneficiaries (median age, 74 years; 57.75% women) aged 65 and above. Among all patients included, researchers observed 29,730 stroke cases from 2016 to 2017; 34,518 from 2017 to 2018; and 36,869 from 2018 to 2019. Regarding the primary outcome of the influenza vaccine’s impact on stroke risk, they only found an association for HS during the 22- to 42-day window, highlighting a lack of significant association between the incidence of stroke and influenza vaccination.1
“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,” wrote authors of the research letter.
Despite previous research showing staggered evidence of the flu vaccine’s effect on CVD, study results confirm that the influenza vaccination is the safest option for staying protected from the disease despite a patients’ comorbidities, including CVD or risk of stroke. With flu season currently heading toward full tilt, the CDC also encouraged all individuals receive the vaccine unless they are below the age of 6 months or have severe allergies triggered by any vaccine ingredient.3
With several health care experts and initiatives focusing on spreading the word about immunization and helping communities stay protected against influenza, this research is yet another example of experts attempting to further stretch the reach of the flu vaccine’s benefits.
“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,” they concluded.
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