title: Influenza vaccination and risk of stroke: Self-controlled case-series study.
abstract: BACKGROUND
  Stroke may be triggered by respiratory infections, including influenza. Influenza vaccination could therefore reduce risk of stroke. Previous studies of this association have shown conflicting results. We aimed to investigate whether influenza vaccination was associated with reduced risk of stroke.
  
  
  METHODS
  We used a self-controlled case series design. The General Practice Research Database (GPRD) was used to extract records of patients aged 18 years or over recorded with stroke (fatal or non-fatal) from September 2001 to May 2009. Statistical modelling with conditional Poisson regression was employed to compute incidence rate ratios (IRR). The incidence rate of stroke in fixed time periods after influenza vaccination was compared with the incidence rate during a baseline period.
  
  
  RESULTS
  There were 17,853 eligible individuals who received one or more influenza vaccinations and experienced a stroke during the observation period. The incidence of stroke was significantly reduced in the first 59 days following influenza vaccination compared with the baseline period. We found reductions of 55% (IRR 0.45; 95% CI 0.36-0.57) in the first 1-3 days after vaccination, 36% (0.64; 0.53-0.76) at 4-7 days, 30% (0.70; 0.61-0.79) at 8-14 days, 24% (0.76; 0.70-0.84) at 15-28 days and 17% (0.83; 0.77-0.89) at 29-59 days after vaccination. Early vaccination between 1 September and 15 November showed a greater reduction in IRR compared to later vaccination given after mid-November.
  
  
  CONCLUSIONS
  Influenza vaccination is associated with a reduction in incidence of stroke. This study supports previous studies which have shown a beneficial association of influenza vaccination for stroke prevention.
authors: Community and Health Research Unit, University of Lincoln, Lincoln School of Health and Social Care, Faculty of Health and Social Sciences, Brayford Campus, Lincoln LN6 7TS, United Kingdom. Electronic address: zasghar@lincoln.ac.uk.
authors: Zahid
authors: Z
authors: Asghar
authors: Department of Medical Statistics, University of Nottingham, Division of Primary Care, School of Community Health Sciences, Floor 13, Tower Building, Nottingham NG7 2RD, United Kingdom.
authors: Carol
authors: C
authors: Coupland
authors: Community and Health Research Unit, University of Lincoln & Lincolnshire Community Health Services NHS Trust, Lincoln School of Health and Social Care, Faculty of Health and Social Sciences, Brayford Campus, Lincoln LN6 7TS, United Kingdom.
authors: Niroshan
authors: N
authors: Siriwardena
mesh_terms: D000368:Aged; D000369:Aged, 80 and over; D005260:Female; D006801:Humans; D007115:Immunization Schedule; D015994:Incidence; D007252:Influenza Vaccines / Q000008:administration & dosage / Q000009:adverse effects*; D008297:Male; D012306:Risk; D020521:Stroke / Q000453:epidemiology* / Q000209:etiology*; D013997:Time Factors; D014611:Vaccination / Q000009:adverse effects
chemical_list: D007252:Influenza Vaccines
keywords: Influenza; Influenza vaccination; Self-controlled case-series method; Stroke; Transient ischaemic attack