Influenza vaccine guards against ischemic and hemorrhagic strokes: LANCET

CANADA: According to a study published in The Lancet Public Health, those who have recently received an influenza vaccination had a lower risk of stroke than those who have not.

Studies that divided stroke into different categories only indicated a protective effect for patients who had ischemic stroke; other types of strokes did not receive the advantages. As opposed to this, Dr. Jessalyn K. Holodinsky, Department of Clinical Neurosciences, Hotchkiss Brain Institute, and colleagues noted in their study that this advantage was applicable to all forms of stroke.

"The largest effect sizes were identified for acute ischemic stroke and cerebral hemorrhage, with nominal differences between them, even though the relative hazard reduction levels differed by type of stroke," they continued.

A respiratory illness may occur before a stroke or myocardial infarction without any warning. Individuals with heart disease are strongly advised to get vaccinated against the flu since it is linked to a lower risk of myocardial infarction and hospitalization for cardiac disease. There is conflicting evidence regarding whether the protective association for stroke is the same and whether this potential impact varies by age and risk group.

The investigators of this study sought to evaluate the risk of stroke in adults following influenza vaccination.

For this purpose, researchers collected administrative data from the Alberta Health Care Insurance Plan starting on September 30, 2009, or May 15 of the year in which residents were documented as having reached the age of 18, whichever came first. 4,141, 209 persons who had registered for health insurance through the provincial system between September 30, 2009, and December 31, 2018, made up the study sample. 38,126 cases of stroke were reported during the study period, while 1,769,565 (42–73%) people received at least one immunization. Any stroke occurrence, including transient ischaemic attack, transient ischaemic attack, intracerebral hemorrhage, and subarachnoid hemorrhage, was the outcome of relevance.

They used Andersen-Gill Cox models to analyze the risk of any stroke event for people with recent (182 days) influenza vaccination compared to those without recent vaccination, adjusting for age, sex, anticoagulant use, atrial fibrillation, chronic obstructive pulmonary disease, diabetes, hypertension, income quintile, and rural or urban home location. To evaluate whether a risk factor has modified the effect, two-way interaction terms between each individual covariate and vaccination status were used. In order to account for baseline confounders, the relationship between immunization and risk of each kind of stroke was also simulated.

Key findings of the study:

• Stroke risk was considerably decreased by recent influenza vaccination (hazard ratio: 0.77 [95% CI: 0.750-0.793]).
• All forms of stroke showed this connection.
• Each covariate evaluated, with the exception of home location, was found to have an effect modification; nonetheless, vaccination was generally linked to a lower risk of stroke across all ages and risk profiles, with the exception of people without hypertension.

"We have demonstrated a definite indirect benefit of influenza vaccination in lowering the risk of stroke using data collected from the entire population of a province in Canada. This effect persisted across all types of strokes and affected all adults, even those without stroke risk factors," concluded the authors.

The use of influenza vaccination as a public health measure to prevent stroke has to be further investigated in a range of situations.

REFERENCE

Holodinsky JK, Zerna C, Malo S, Svenson LW, Hill MD. Association between influenza vaccination and risk of stroke in Alberta, Canada: a population-based study. Lancet Public Health. 2022 Nov;7(11):e914-e922. doi: 10.1016/S2468-2667(22)00222-5. PMID: 36334607.