Background: There is a need for real-time, within season, estimation of influenza vaccine effectiveness (V/E) in order to optimize management of circulating influenza; a need which will be even greater in a pandemic situation.
Objective: To examine the potential of using routinely collected national virological surveillance data to generate estimates of V/E in real-time.
Methods: Information collected in the integrated clinical and virological community influenza surveillance program of the Royal College of General Practitioners and Health Protection Agency over three winters 2004/5-2006/7 was used. From vaccination and clinical data entered on the investigation request form and the influenza virology detection result, we calculated the odds of vaccination in virus positive and in virus negative persons. One minus the ratio of these odds provided crude estimates of V/E, which were adjusted for confounding using logistic regression.
Results: Multivariate analysis suggested that adjustments were necessary for patient age and month of sampling. The annual adjusted V/E was 2005/6, 67% (95% confidence intervals 41-82); 2006/7 55% (26-73) and 2007/8 67% (41-82); and in persons <65 years, 70% (57-78) and 65 years and over 46% (-17-75). Estimates derived separately for influenza A and B, for interval between illness onset and swab sample, for early and late winter periods, and according to viral load did not differ significantly in each comparison.
Conclusion: We have demonstrated the potential of using routine management data to provide estimates of V/E in real-time. We recommend this approach to V/E measurement in the evaluation of national influenza vaccination programmes.