Influenza Vaccination Provides Indirect Benefit for Stroke Prevention

Recent influenza vaccination may have a protective effect against stroke, according to results of a population-based study published in The Lancet Public Health.

Globally, stoke is the third leading cause of morbidity and mortality, however, its cause can be difficult to determine. Despite its sometimes-elusive cause, infectious diseases including respiratory illnesses are a known risk factor. This study was designed to evaluate whether influenza vaccination may reduce stroke risk.

Researchers from the University of Calgary sourced data for this study from the Alberta Health Care Insurance Plan (AHCIP) in Canada. Among all registered individuals (N=4,141,209) between 2009 and 2018, incidence of stroke was evaluated on the basis of receiving an influenza vaccine.

A total of 2,371,644 individuals never received an influenza vaccine and 1,769,565 had been vaccinated. The unvaccinated and vaccinated cohorts were aged mean 38.98 (standard deviation [SD], 17.31) and 48.34 (SD, 19.17) years, 44.41% and 55.98% were women, and 77.85% and 80.87% lived in a rural area, respectively.

A total of 38,126 stroke events occurred during the study period, of which 90.41% were first stroke events. Stratified by vaccination, the vaccinated group had higher:

• crude rates of any stroke event (1.25% vs 0.52%),
• acute ischemic stroke (0.85% vs 0.35%),
• intracranial hemorrhage (0.11% vs 0.06%),
• subarachnoid hemorrhage (0.06% vs 0.03%), and
• transient ischemic attack (0.28% vs 0.09%) compared with the unvaccinated cohort, respectively.

Although crude rates were elevated, vaccination was associated with decreased risk for:

• intracranial hemorrhage (adjusted hazard ratio [aHR], 0.730; 95% CI, 0.677-0.787), acute ischemic stroke (aHR, 0.742; 95% CI, 0.722-0.763),
• subarachnoid hemorrhage (aHR, 0.803; 95% CI, 0.716-0.901), and
• transient ischemic attack (aHR, 0.915; 95% CI, 0.870-0.963) after adjusting for age, gender, comorbidities, and socioeconomic status.

In a subgroup analysis, vaccination was significantly favored over no vaccination for protecting against stroke risk (all P <.001), except for patients without hypertension (hazard ratio [HR], 1.146; P <.001).

In general, the protective effect was greater among men than women (P <.001); patients with chronic obstructive pulmonary disease (P <.001), diabetes (P <.001), or atrial fibrillation (P =.001) compared with patients without those comorbidities; anticoagulant users compared with nonusers (P <.001), and individuals in the lowest four quintiles of income compared with the highest quintile (P =.017).

Among the subset of individuals with available data for the entire study period (n=2,407,425), a significant interaction was observed for the protective effect against stroke and the number of influenza vaccines received (P <.001), in which receiving vaccines every year associated with decreased stroke risk (HR, 0.432; 95% CI, 0.309-0.604) whereas those who only received 1 vaccination were not (HR, 1.082; 95% CI, 0.931-1.258).

This study may have been limited, as the biological mechanism of protection could not be evaluated.

These data indicated that a clear indirect protective effect against stroke from influenza vaccination was observed.

The researchers concluded that “Further studies in a variety of settings are needed to evaluate whether influenza vaccination could be used as a public health strategy to prevent stroke.”

Disclosure: Multiple authors declared affiliations with industry. Please refer to the original article for a full list of disclosures.

This article originally appeared on Neurology Advisor