Study Objective To estimate the effectiveness of influenza vaccination in people in the United Kingdom at low risk for influenza during an 11-year period from July 1996-June 2007 Design Case-control study Data Source United Kingdom General Practice Research Database Subjects A total of 4985 patients (aged < 80 yrs) who were considered to be at low risk for influenza, but were diagnosed with influenza or influenza-like illness from July 1996-June 2007, were identified as cases A total of 19,940 controls who were also low-risk individuals, who had no diagnosis of influenza, were matched to the cases by year of birth, sex, specific general practice, and date on which case patient was diagnosed with influenza (index date) Measurements and Main Results We identified previous vaccination status from the subjects' electronic medical records and evaluated the effect of timing of influenza vaccination and the number of previous annual vaccinations received We estimated the relative risk of influenza in those who received the influenza vaccination compared with those who did not, conditional on the matching variables Odds ratios (ORs) and 95% confidence intervals (CIs) were calculated Age, sex, general practice, index date, and preexisting medical conditions were closely controlled for in the analysis Vaccination conferred an important protective effect (OR 0 74, 95% CI 0 60-0 91) when given within 4 months before seasonal influenza outbreaks This effect was similar for each calendar year and across all ages Conclusion Seasonal influenza vaccination provided substantial protection against clinically diagnosed influenza when given within 4 months of annual seasonal outbreaks in people at low risk A review of the incidence of influenza over the 11-year period provides compelling indirect evidence that influenza vaccines were similarly effective in those who were at high risk These results provide a useful background to current and future influenza outbreaks whose course is uncertain
