Individual and Joint Effects of Influenza-Like Illness and Vaccinations on Stroke in the Young: A Case-Control Study

Background: Influenza-like illness (ILI) is an acute trigger for stroke, although joint effects of vaccinations and ILI have not yet been explored. Methods: Data for our case-control study was obtained from MarketScan Commercial Claims and Encounters between 2008 and 2014. Patients 18 to 65 years old who experienced a stroke were matched on age and admission date to a control, defined as patients with head trauma or ankle sprain at an inpatient or emergency department visit. Exposures were ILI in the prior 30 days, and any type of vaccination during the year prior. Our outcome was ischemic and intracerebral hemorrhagic strokes identified using International Classification of Diseases, Ninth Revision (ICD-9) codes. Logistic regression models estimated adjusted odds ratios (aORs) controlling for preventive care visits, diabetes, valvular heart disease, smoking, alcohol abuse, obesity, and hypertension. Results: We identified and matched 24 103 cases 18 to 44 years old and 141 811 45 to 65 years old. Those aged 18 to 44 years had increased stroke risk 30 days after ILI (aOR, 1.68 [95% CI, 1.51-1.86]) and reduced risk with any vaccination in the year prior (aOR, 0.92 [95% CI, 0.87-0.99]). Joint effects indicate that ILI was associated with increased stroke risk among those with (aOR, 1.41 [95% CI, 1.08-1.85]) and without (aOR, 1.73 [95% CI, 1.55-1.94]) vaccinations in the prior year (P-interaction=0.16). Among those aged 45 to 65 years, adjusted analyses indicate increased stroke risk for those with ILI (aOR, 1.32 [95% CI, 1.26-1.38]), although there was no effect of vaccinations (aOR, 1.00 [95% CI, 0.97-1.02]). Joint effects indicate that ILI was not associated with stroke among those with any vaccination (aOR, 1.07 [95% CI, 0.96-1.18]) but was associated with increased risk among those without vaccinations ([aOR, 1.39 [95% CI, 1.32-1.47]; P-interaction<0.001). Conclusions: ILI 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. Joint effects of ILI and vaccinations indicate vaccinations can reduce the effect of ILI on stroke.