Estimating influenza vaccine effectiveness using routinely collected laboratory data.

BACKGROUND

Estimation of influenza vaccine effectiveness (V/E) is needed early during influenza outbreaks in order to optimise management of influenza--a need which will be even greater in a pandemic situation.

OBJECTIVE

Examine the potential of routinely collected virological surveillance data to generate estimates of V/E in real-time during winter seasons.

METHODS

Integrated clinical and virological community influenza surveillance data collected over three winters 2004/5-2006/7 were used. We calculated the odds of vaccination in persons that were influenza-virus-positive and the odds in those that were negative and provided a crude estimate of V/E. Logistic regression was used to obtain V/E estimates adjusted for confounding variables such as age.

RESULTS

Multivariable analysis suggested that adjustments to the crude V/E estimate were necessary for patient age and month of sampling. The annual adjusted V/E was 2005/6, 67% (95% CI 41% to 82%); 2006/7 55% (26% to 73%) and 2007/8 67% (41% to 82%). The adjusted V/E in persons <65 years was 70% (57% to 78%) and 65 years and over 46% (-17% to 75%). Estimates differed by small insignificant amounts when calculated separately for influenza A and B; by interval between illness onset and swab sample; by analysis for the period November to January in each year compared with February to April and according to viral load.

CONCLUSION

We have demonstrated the potential of using routine virological and clinical surveillance data to provide estimates of V/E early in season and conclude that it is feasible to introduce this approach to V/E measurement into evaluation of national influenza vaccination programs.