Flu vaccination linked to a substantial short-term reduction in stroke risk, new study finds
A major study from the University of Lincoln, funded by the National Institute for Health Research and published in the journal Vaccine, found that receiving an influenza (flu) vaccine was associated with a significantly lower risk of suffering a first stroke for up to two months after vaccination. The research shows that the risk of stroke fell by roughly one fifth during the first 59 days following the flu jab.
The protective effect was strongest when vaccines were given earlier in the influenza season. In the first week after vaccination, there were 36 percent fewer stroke events than expected in a comparable baseline period; the second week showed a 30 percent reduction. Weeks three and four recorded a 24 percent reduction, and risk remained reduced by about 17 percent between days 29 and 59 after vaccination.
Researchers note a plausible biological explanation: respiratory infections such as influenza can trigger cardiovascular events, and preventing influenza through vaccination may therefore lower the likelihood of related conditions, including stroke. The antibody response to seasonal influenza vaccine typically lasts several months, which corresponds with the observed window of reduced stroke incidence.
The team used a self-controlled case series design to examine almost 18,000 people aged 18 or older who experienced a first stroke between 2001 and 2009. This method compares risk during specified time periods after vaccination with risk during other periods for the same individuals, reducing confounding by fixed patient characteristics. Overall there were 17,853 eligible individuals who had one or more influenza vaccinations and experienced a stroke during the observation period.
The study reports marked reductions in stroke incidence during the immediate period after vaccination: a 55 percent reduction in the first 1–3 days (IRR 0.45; 95% CI 0.36–0.57), 36 percent at 4–7 days (IRR 0.64; 95% CI 0.53–0.76), 30 percent at 8–14 days (IRR 0.70; 95% CI 0.61–0.79), 24 percent at 15–28 days (IRR 0.76; 95% CI 0.70–0.84), and 17 percent at 29–59 days (IRR 0.83; 95% CI 0.77–0.89). Vaccination given earlier in the season (between September 1 and November 15) showed a greater reduction in stroke incidence compared with vaccination administered later.
Lead author Niro Siriwardena, Professor of Primary and Pre-Hospital Health Care and a practicing GP, said the findings are important and, if confirmed in clinical trials, could have meaningful implications for public health. He and his colleagues — including statisticians Zahid Asghar of the Community and Health Research Unit at the University of Lincoln and Dr. Carol Coupland of the University of Nottingham — emphasize that the results support current recommendations for vaccinating people at high risk, while highlighting an additional potential benefit: stroke prevention.
“Our analysis indicates that earlier vaccination in the influenza season is linked to a greater reduction in stroke risk, which reinforces the public health message to vaccinate early,” Professor Siriwardena commented. The researchers are now planning further studies to assess whether expanding vaccination recommendations to include younger adults at elevated stroke risk could be justified if a causal relationship is established.
This study builds on prior research that associated influenza vaccination with lower risks of stroke and first heart attack. Using the self-controlled case series approach helped minimize potential biases and strengthened the association between vaccination and reduced stroke incidence, though observational studies cannot by themselves prove causation.
Source: Cerri Evans – University of Lincoln
Image source: Image adapted from the University of Lincoln press release
Original research: Abstract for “Influenza vaccination and risk of stroke: Self-controlled case-series study” by Zahid Asghar, Carol Coupland, and Niroshan Siriwardena in Vaccine. Published online October 5, 2015. DOI: 10.1016/j.vaccine.2015.08.013
Abstract
Influenza vaccination and risk of stroke: Self-controlled case-series study
Background
Respiratory infections such as influenza can trigger cardiovascular events, including stroke. Influenza vaccination might therefore reduce stroke risk. Previous studies have reported mixed findings; this study aimed to clarify the association.
Methods
The authors applied a self-controlled case series design to records drawn from the General Practice Research Database (GPRD). They identified adults aged 18 and over with recorded fatal or non-fatal stroke events between September 2001 and May 2009. Conditional Poisson regression models were used to compute incidence rate ratios (IRRs), comparing stroke incidence in defined intervals after influenza vaccination with a baseline period.
Results
Among 17,853 individuals who received one or more influenza vaccinations and experienced a stroke during observation, stroke incidence was significantly lower in the first 59 days after vaccination versus baseline. The reductions were largest in the earliest days after vaccination and remained statistically significant through 29–59 days. Early-season vaccination produced a stronger reduction in IRR than vaccination administered later.
Conclusions
Influenza vaccination was associated with a reduced incidence of stroke in this large self-controlled case-series study. These findings support earlier studies that observed a beneficial association between influenza vaccination and stroke prevention, and they underline the potential public health value of timely seasonal flu immunization.