The risk of stroke was about 23% lower in the 6 months following a flu shot, regardless of the patient’s age, sex, or underlying health conditions.
“There is an established link between upper respiratory infection and both heart attack and stroke. This has been very salient in the past few years throughout the COVID-19 pandemic,” study author Jessalyn Holodinsky, PhD, a stroke epidemiologist and postdoctoral fellow in clinical neurosciences at the University of Calgary (Alta.) told this news organization.
“It is also known that the flu shot can reduce risk of heart attack and hospitalization for those with heart disease,” she said. “Given both of these [observations], we thought it prudent to study whether there is a link between vaccination for influenza and stroke.”
The study was published in the Lancet Public Health.
Large effect size
The investigators analyzed administrative data from 2009 through 2018 from the Alberta Health Care Insurance Plan, which covers all residents of Alberta. The province provides free seasonal influenza vaccines to residents under the insurance plan.
The research team looked for stroke events such as acute ischemic stroke, intracerebral hemorrhage, subarachnoid hemorrhage, and transient ischemic attack. They then analyzed the risk of stroke events among those with or without a flu shot in the previous 6 months. They accounted for multiple factors, including age, sex, income, location, and factors related to stroke risk, such as anticoagulant use, atrial fibrillation, chronic obstructive pulmonary disease, diabetes, and hypertension.
Among the 4.1 million adults included in the researchers’ analysis, about 1.8 million (43%) received at least one vaccination during the study period. Nearly 97,000 people received a flu vaccine in each year they were in the study, including 29,288 who received a shot in all 10 flu seasons included in the study.
About 38,000 stroke events were recorded, including about 34,000 (90%) first stroke events. Among the 10% of strokes that were recurrent events, the maximum number of stroke events in one person was nine.
Overall, patients who received at least one influenza vaccine were more likely to be older, be women, and have higher rates of comorbidities. The vaccinated group had a slightly higher proportion of people who lived in urban areas, but the income levels were similar between the vaccinated and unvaccinated groups.
The crude incidence of stroke was higher among people who had ever received an influenza vaccination, at 1.25%, compared with 0.52% among those who hadn’t been vaccinated. However, after adjusting for age, sex, underlying conditions, and socioeconomic status, recent flu vaccination (that is, in the previous 6 months) was associated with a 23% reduced risk of stroke.
The significant reduction in risk applied to all stroke types, particularly acute ischemic stroke and intracerebral hemorrhage. In addition, influenza vaccination was associated with a reduced risk across all ages and risk profiles, except patients without hypertension.
“What we were most surprised by was the sheer magnitude of the effect and that it existed across different adult age groups, for both sexes, and for those with and without risk factors for stroke,” said Dr. Holodinsky.
Vaccination was associated with a larger reduction in stroke risk in men than in women, perhaps because unvaccinated men had a significantly higher baseline risk for stroke than unvaccinated women, the study authors write.