Influenza vaccination and risk of stroke: self-controlled case-series study

Background Previous studies suggest that stroke may be triggered by respiratory infections including influenza. Influenza vaccination could therefore reduce risk of stroke but previous studies may have been affected by residual confounding. We aimed to investigate whether influenza vaccination was associated with risk of stroke using the self-controlled case series method. Methods We used a self-controlled case series design. The Clinical Practice Research Datalink (CPRD) was used to extract records of patients aged 18 years or over recorded with a first stroke (fatal or non-fatal) occurring between September 2001 to August 2009. Statistical modelling with conditional Poisson regression was used to compute incidence rate ratios (IRR). The incidence rates of stroke during fixed time periods following vaccination were compared with the incidence rate during a baseline period. Results We included 21256 cases with stroke in the analysis. The incidence of stroke was significantly reduced in the first 180 days following influenza vaccination compared with the baseline period. We found a reduction of 41% (IRR 0.59; 95% CI 0.53-0.65) in the first 14 days after vaccination, and 20% (0.80; 0.72-0.88) 23% (0.77, 0.72-0.83), 15% (0.85, 0.79-0.91), 13% (0.87, 0.81-0.94) and 9% (0.91, 0.86-0.95) at 15-28 days, 29-59 days, 60-90 days, 91-120 days and 121-180 days after vaccination respectively. There were no differences in the association by age group (above or below 65 year) or sex. Conclusions Influenza vaccination is associated with a reduction in incidence of stroke. This study supports a beneficial association of influenza vaccination on stroke prevention as reported by another study using a different method of study [1]. [1] Siriwardena AN, Asghar Z, Coupland C. Influenza and pneumococcal vaccination and risk of stroke or transient ischaemic attack—Matched case control study. Vaccine 2014; 32: 1354-136