We investigated whether there is an increased racial disparity in stroke incidence during flu season.

The increased risk of ischemic stroke in black versus white U.S. residents is not fully explained by known risk factors. Influenza has been implicated as a risk factor, and studies show that blacks receive influenza vaccination less often than whites.

We performed a retrospective cohort study using inpatient and outpatient claims between 2008-2015 from a nationally representative 5% sample of Medicare beneficiaries. We included patients of black or white race who were ≥66 y.o. The outcomes were ischemic stroke outside of influenza season versus during influenza season, defined as December through March. We calculated incidence rates and used Cox regression models adjusted for demographics and vascular risk factors to examine the association between race and ischemic stroke.

Among 1,511,393 white and 138,026 black beneficiaries, blacks were slightly younger (mean age, 72.7 vs. 73.5), more often female (60.5% vs. 56.9%), and less likely to have atrial fibrillation but more likely to have hypertension, diabetes, heart failure, and chronic kidney disease. During 4.7 (±2.2) years of follow-up, 61,188 patients experienced a stroke. During influenza season, stroke incidence was 1.08 (95% CI, 1.02-1.11) per 100 person-years in blacks versus 0.72 (95% CI, 0.69-0.72) per 100 person-years in whites. Outside of influenza season, stroke incidence was 1.22 (95% CI, 1.19-1.26) per 100 person-years in blacks versus 0.80 (95% CI, 0.80-0.81) per 100 person-years in whites. Blacks had an increased hazard of stroke both during influenza season (HR, 1.58; 95% CI, 1.52-1.66) and outside of influenza season (HR, 1.58; 95% CI, 1.53-1.63).

In a cohort of elderly U.S. patients, we found that the black-white disparity in stroke risk was not more pronounced during influenza season. Our findings do not suggest that influenza contributes to the black-white disparity in stroke risk.