Influenza-like illness is associated with an increased risk of stroke in young and middle-aged adults, but being vaccinated may lower the risk of these odds. These are the findings of a study published in Stroke.
Influenza-like illness is considered an acute trigger for stroke. As much as 14% of ischemic strokes in the United States occur in adults aged 18-45 years. Risk reduction strategies successful in older populations may not be as beneficial in this younger population due to different causes of stroke, researchers explained. For the study, the researchers sought to assess the relationship between influenza-like illness and any type of vaccination, and the combined effects of vaccinations and influenza-like illness on risk for stroke in both populations.
They carried out a case-control study using MarketScan Commercial Claims and Encounters between 2008 and 2014 dataset of patients 18-65 years of age who experienced stroke and matched on admission date and age to a control patient with either head trauma or ankle sprain, inpatient or emergency department visit. The factors that were associated with outcomes of interest were any type of vaccination in the previous year and influenza-like illness in the previous 30 days. The study outcome was ischemic and intracerebral hemorrhagic strokes.
Among the younger population (18-44 years of age), the researchers found and matched 24,103 cases, and among the older population (45-65), 141,811 cases. Increased risk of stroke within 30 days of influenza-like illness was seen in the younger population (adjusted odds ratio [aOR], 1.68; 95% CI, 1.51–1.86) and reduced risk of stroke with any vaccination within the prior year (aOR, 0.92; 95% CI, 0.87–0.99).
Increased risk for stroke associated with influenza-like illness was seen in those with vaccinations in the prior year (aOR, 1.41; 95% CI, 1.08–1.85), and in those without vaccinations in the prior year (aOR, 1.73; 95% CI, 1.55-1.94). Increased risk for stroke was seen in the older population with influenza-like illness (aOR, 1.32; 95% CI, 1.26-1.38), and vaccinations had no effect (aOR, 1.00; 95% CI, 0.97-1.02).
Analysis of joint effects revealed that risk for stroke was not associated with influenza-like illness among patients with any vaccination (aOR, 1.07; 95% CI, 0.96-1.18), but was linked to increased risk for stroke among unvaccinated patients (aOR, 1.39; 95% CI, 1.32-1.47).
Study limitations included the possible misclassifications of disease severity, vaccinations, and preventive care visits, inability to distinguish influenza vaccinations, unmeasured cofounders, and a lack of representation of uninsured individuals or those with insurance types not captured by MarketScan.
According to the researchers, “[Influenza-like illness] was associated with increased stroke risk in the young and middle-aged, while vaccinations of any type were associated with decreased risk among the young.” The effect of [influenza-like illness] on stroke can be reduced by any type of vaccination in the previous year.
Vollmer BL, Solowey J, Chen X, et al. Individual and joint effects of influenza-like illness and vaccinations on stroke in the young: a case-control study. stroke Published online July 11, 2022. doi:10.1161/STROKEAHA.121.038403