## 3 Results
### The Association between the Risk of Stroke and the Influenza Vaccination for Different Age and Sex  
The total follow-up period was 177,838.0 and 201,118.5 person years for the unvaccinated and vaccinated patients, respectively, and the risk of stroke in both groups of subjects in this study is shown in Table 2. Propensity scores were calculated after adjusting for age, sex, Charlson comorbidity index, diabetes, dyslipidemia, atrial fibrillation, antihy-perstinives, diuretics, beta blocking agents, calcium channel blockers, renin-angiotensin, statins, metformin, aspirin, residential area and monthly income. The adjusted HRs (aHRs) and 95% confidence intervals (CIs) of stroke in vaccinated subjects were significantly lower during all seasons when compared to those in the unvaccinated subjects (aHRs [95% CI] of all seasons: 0.58 [0.56, 0.61]). During non-influenza season, the risk reduction was potentially more profound than during the influenza season (aHRs [95% CI] = 0.60 [0.56, 0.63] and 0.56 [0.52, 0.60] during influenza and non-influenza season, respectively). The same results could be found in all strata of age. The stratified analysis revealed that aHRs decreased significantly in the vaccinated subjects during the influenza season, non-influenza season and all seasons, in the subjects aged between 55 to 64 and 65 years and older. Among patients with a relative younger age, the risk reduction was potentially lower compared with those aged more than 65 years (aHRs [95% CI] = 0.53 [0.48, 0.58], 0.44 [0.38, 0.50], 0.49 [0.46, 0.53] in patients age between 55 to 64 during the influenza and non-influenza season, and all seasons, respectively; aHRs [95% CI] = 0.61 [0.57, 0.66], 0.58 [0.53, 0.63], 0.60 [0.57, 0.63] in patients age \(\geq\) 64 years during influenza and non-influenza season, and all seasons, respectively) (Table 2). Both female and male patients had a significant lower risk of stroke occurrence after receiving vaccination. The risk reduction was potentially more significant among male patients (aHRs [95% CI] = 0.63 [0.58, 0.68], 0.58 [0.52, 0.64], 0.61 [0.57, 0.65] during influenza season, non-influenza season and all seasons among female patients; aHRs [95% CI] = 0.57 [0.53, 0.62], 0.53 [0.48, 0.59], 0.56 [0.52, 0.59] during influenza season, non-influenza season and all seasons among male patients) (Table 2). Sensitivity Analysis of Stroke Risk Reduction after Different Times of Vaccination during Influenza Season, Non-Influenza Season and All Seasons  
Influenza vaccination is not only significantly associated with stroke risk reduction in every stratum of the cohort, the effect is also dose dependent. Tables 3-5 contain the aHRs of vaccination in the risk reduction of stroke during the influenza, non-influenza and all seasons. There is a marked trend of lowering aHRs in subjects obtaining one to four vaccinations as compared to those unvaccinated, either in the whole cohort or in every stratum (Tables 3-5). Regardless of the underlying co-morbidities and medication history, the risk of stroke decreased after receiving the influenza vaccination more than two times and the trend was significant during the influenza season (Table 3). The neuroprotectiveeffect after multiple times of influenza vaccination was also observed during the noninfluenza season and all seasons (Tables 4 and 5).