Strokes are associated with a well-known set of risk factors, such as increasing age, hypertension, smoking, diabetes and heart disease. Yet these triggers only explain 60% of strokes. The causes of the rest--and reasons strokes peak in winter--are unclear, but a growing body of evidence suggests flu vaccines may help to prevent these cases.
Multiple studies have linked flu vaccines to decreased risk of heart attack and stroke, but doubts about the association remain. The latest study is unlikely to dismiss these doubts but nonetheless strengthens the case for randomized controlled trials investigating the link between flu vaccines and stroke risk. Writing in Vaccine, the British team behind the latest study describe how their mining of a database of healthcare records suggests people who receive the seasonal flu vaccine are 24% less likely to suffer from a stroke.
The researchers reached the 24% figure by analyzing the records of 47,000 patients who had suffered a stroke--or the "mini-stroke" known as transient ischemic attack (TIA)--between 2001 and 2009. Each of these patients was then matched to a person of the same age and gender who visited the physician around the same time to create a control group. In the control group, 50.8% of patients received the flu vaccine in the year their matched case suffered a stroke. Among stroke victims the vaccination rate was 50.6%.
No link was found between the flu vaccine and TIA, or between the pneumococcal vaccine and either form of stroke. These negative results strengthen the researchers' belief that the flu shot is having an effect. If another factor was responsible for the decreased risk of stroke--such as the fact that healthy people are more likely to get vaccinated--the researchers would have expected to see the trend in TIA and the pneumococcal vaccine. Efforts were made to adjust for factors like the "healthy vaccine" bias and potential triggers of a stroke.