Following on from a recent study that suggested undergoing yearly vaccination for influenza can greatly reduce Alzheimer's risk, researchers here show that influenza vaccination correlates with a lower risk of stroke. The mechanisms of interest behind both of these correlations seem likely to revolve around chronic inflammation, an important factor in both the growth of atherosclerotic plaques in blood vessels and the onset and progression of neurodegenerative conditions. Firstly, suffering influenza is an inflammatory event, and the vaccine lowers the incidence and severity of that outcome. Secondly vaccination of this sort can reduce inflammation in the central nervous system via what is known as trained immunity, an improvement in the function of the innate immune system in response to the vaccine.
Researchers looked at a health care database in Spain and identified people who were at least 40 years old and had a first stroke over a 14-year period. Each person who had a stroke was compared to five people of the same age and sex. There were 14,322 people who had a stroke and 71,610 people who did not have a stroke. Then the researchers looked at whether people had received the influenza vaccine at least 14 days before the stroke or before that same date for those who did not have a stroke.
A total of 41.4% of those who had a stroke had received the flu shot, compared to 40.5% of those who did not have a stroke. But the people who got the shot were more likely to be older and to have other conditions such as high blood pressure and high cholesterol that would make them more likely to have a stroke. Once researchers adjusted for those factors, they found that those who received a flu shot were 12% less likely to have a stroke than those who did not.
The researchers also looked at whether the pneumonia vaccine had any effect on the risk of stroke and found no protective effect. Since the study was observational, it does not prove that getting the flu shot reduces the risk of stroke. It only shows an association. There could be other factors that were not measured that could affect the risk of stroke.