A study from CaHRU, ‘Influenza vaccination and risk of stroke: self-controlled case-series study‘, recently published in the journal Vaccine, received international coverage in news media. The study of almost 18,000 patients with stroke looked at data from general practices in England using a self-controlled case series analysis, and showed that the risk of suffering a stroke is significantly reduced for up to two months after receiving a flu vaccine.
The lead author was Dr Zahid Asghar, who conducted the research together with Prof Niro Siriwardena (both from CaHRU) and Dr Carol Coupland (from Nottingham University School of Community Health Sciences). This is the fourth in a series of studies over the past 10 years looking at the link between flu vaccination and reduction in risk of heart attack and stroke.
The coverage included articles in The Times, Sun and Scottish Sun newspapers, BBC television and radio and online media outlets in the US, Middle East, Asia, Australasia and South America. The team are now designing future studies to explore the potential for flu vaccination to reduce risk of heart attack and stroke.
The latest study from CaHRU is published online in Vaccine this month. The study, ‘Influenza vaccine and risk of stroke: self-controlled case series study‘, was co-authored by Dr Zahid Asghar and Professor Niro Siriwardena from CaHRU together with Carol Coupland, professor of medical statistics at Nottingham University. The study used a self-controlled case series design with data from the General Practice Research Database (GPRD) on adult patients aged 18 years and above with fatal or non-fatal stroke during 8 years from September 2001 to May 2009.
Statistical analysis was used to compute incidence of stroke after flu vaccination compared with incidence during a baseline period in almost 18000 people who received one or more influenza vaccinations and experienced a first stroke during the observation period. The incidence (incidence rate ratio: IRR) of stroke was significantly reduced in the first 59 days following flu vaccination compared with the baseline period. We found reductions in stroke incidence of 55% in the first 1–3 days after vaccination, 36% at 4–7 days, 30% at 8–14 days, 24% at 15–28 days and 17% at 29–59 days after vaccination. Early vaccination between 1 September and 15 November was associated with a greater reduction in incidence of stroke compared to later vaccination given after mid-November.
Flu vaccination was associated with a reduction in incidence of stroke, which confirms findings from previous studies showing possible protective effects of flu vaccination for both heart attack and stroke. This association needs further confirmation with experimental (randomised controlled) trials.
Dr Zahid Asghar recently attended the North American Primary Care Research Group (NAPCRG) conference in New York City to present findings from ‘Influenza vaccination and risk of stroke: self-controlled case-series study’. This was an observational study of investigating the association between influenza vaccination and stroke led by Professor Niro Siriwardena, director of CaHRU, and Dr Carol Coupland, associate professor in statistics at the University of Nottingham.
The study involved analysis of almost 18 thousand cases of stroke over a period of eight years from a general practice database, the Clinical Practice Research Datalink. The investigators found a significant reduction in risk of stroke up to 59 days following vaccination. The work follows publication of a case-control study earlier this year and involving almost 10,000 patients showing a 20% reduction in risk of stroke associated with flu vaccination together with previous studies conducted by Professor Siriwardena and his team showing a reduction in risk of heart attack associated with influenza vaccine.
It is not known how influenza vaccination prevents heart attack or stroke. It might be because influenza has been shown to occur two to four weeks before these conditions and may trigger them in a proportion of cases or it might be due to immunological protection from the vaccine. There was worldwide interest in this area of research following the team’s study published in the Canadian Medical Association Journal.
A further study, published in the Journal of Infectious Diseases, points to a link between influenza as a trigger of cardiovascular events, while presentations given in the past week in Toronto suggest that influenza vaccination might prevent such events.