Influenza vaccination is associated with a lower risk of stroke among adults, even if they are not at high risk for stroke, according to new research.
The risk of stroke was reduced by 23% in the 6 months after the flu shot, regardless of patient age, gender or underlying health conditions.
“There is an established link between upper respiratory infection and both heart attack and stroke. This has been very evident in recent years through the COVID-19 pandemic,” study author Jessalyn Holodinsky, PhD, an epidemiologist and postdoctoral fellow in clinical neuroscience at the University of Calgary, Alberta, Canada, said Medscape Medical News.
“The flu shot is also known to reduce the risk of heart attack and hospitalization for those with heart disease,” she said. “Considering both [observations]we thought it wise to investigate whether there is a link between influenza vaccination and stroke.”
The study was published on November 1 Lancet Public Health.
Large effect size
The researchers analyzed administrative data from 2009 to 2018 from the Alberta Health Care Insurance Plan, which covers all Alberta residents. The province provides free seasonal flu shots to residents under the insurance program.
The research team looked for stroke events such as acute ischemic stroke, intracerebral haemorrhage, subdural haemorrhage and transient ischemic attack. They then analyzed the risk of stroke among those with or without a flu shot in the previous 6 months. They accounted for many factors, including age, sex, income, location, and factors associated with stroke risk, such as blood-thinning medication 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 shot each year they were in the study, including 29,288 who received a shot during all 10 flu seasons covered by the study.
About 38,000 stroke cases were recorded, of which about 34,000 (90%) were first stroke cases. Among the 10% of strokes that were recurrent events, the maximum number of strokes in a single subject was nine.
Overall, patients who received at least one influenza vaccine were more likely to be older, to be female, and to have higher rates of comorbidities. The vaccinated group had a slightly higher proportion of people living in urban areas, but income was similar between the vaccinated and unvaccinated groups.
The net rate of stroke was higher among people who had ever received an influenza vaccination, at 1.25%, compared to 0.52% among those who had never been vaccinated. However, after adjusting for age, sex, underlying medical conditions, and socioeconomic status, recent influenza vaccination (ie, within the last 6 months) was associated with a 23% lower risk of stroke.
A significant reduction in risk was observed for all types of stroke, particularly acute ischemic stroke and intracerebral haemorrhage. In addition, influenza vaccination was associated with reduced risk in all ages and risk profiles, except in nonhypertensive patients.
“What surprised us the most was the magnitude of the effect and that it was present across different age groups of adults, for both sexes, and for those with and without risk factors for stroke,” Holodinsky said.
Vaccination was associated with a greater reduction in stroke risk in men than in women, perhaps because unvaccinated men had a significantly higher baseline risk of stroke than unvaccinated women, the study authors write.
To promote cardiovascular health
In addition, vaccination was associated with greater relative reductions in stroke risk in younger age groups, lower income groups, and those with diabetes, COPD, and anticoagulant use.
Among the 2.4 million people followed throughout the study period, vaccination coverage increased with the number of vaccines received. People who received a series of vaccinations each year had a significantly lower risk of stroke than those who received a single injection.
Holodinsky and colleagues are conducting further research on influenza vaccination, including the risk of stroke in children. They are also investigating whether the reduced risk applies to other vaccinations against respiratory diseases, such as COVID-19 and pneumonia.
“We hope this increased vaccination effect will encourage more adults to get a flu shot,” she said. “One day, vaccinations may be considered a key pillar of cardiovascular health, along with diet, exercise, control of high blood pressure and high cholesterol, and smoking cessation.”
Future research should also examine the reasons why adults — especially high-risk people with underlying medical conditions — don’t get the recommended flu shots, the study authors wrote.
“Call to action“
Reviews of results for Medscape Medical News, Bahar Behrouzi, a PhD student focused on clinical epidemiology at the Institute of Health Policy, Management and Evaluation, University of Toronto, Ontario, Canada, said: “There are multiple observational studies around the world that show that influenza vaccine uptake is low among the general public and individuals who are at high risk. As we investigate these questions, our hope is that we can continue to build confidence in respiratory viral vaccines such as influenza vaccines by continuing to generate rigorous evidence with the latest data.”
Behrouzi, who was not involved in this study, has researched flu vaccination and cardiovascular disease. She and her colleagues have found that flu vaccines were associated with a 34% lower risk of serious adverse cardiovascular events, including a 45% lower risk among patients with a recent acute coronary syndrome.
“The broader public health message is for people to take action and get the seasonal flu shot, especially if they’re part of a high-risk group,” she said. “In our research, we have framed this message as a call to action not only for the general public, but also for healthcare professionals – especially specialists such as cardiologists or neurologists – to encourage or remind them to engage in a conversation about the broad benefits of vaccination.” beyond preventing or reducing the severity of influenza infection.”
The study was conducted without external funding. Holodinsky and Behrouzi have reported no relevant disclosures.
Lancet Public Health. Published November 1, 2022. Full text
Carolyn Crist is a health and medical journalist who reports on the latest research for Medscape, MDedge, and WebMD.
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