Effectiveness of influenza vaccination.

To the Editor: In their article about the effective­ ness of influenza vaccination in the community­ dwelling elderly (Oct. 4 issue),1 Nichol and col­ leagues define the influenza season as the time from the reporting of the first isolate to the re­ porting of the last isolate, for each region and season. We are concerned that their analysis in­ cluded deaths early and late in the season that were almost certainly unrelated to influenza and that could not have been prevented by vaccination. The actual period of elevated risk for influenza­ related death is substantially shorter than the in­ terval between the reporting of the first isolate and that of the last isolate. In two West Coast cities, the period of predominant influenzavirus transmission accounted for less than half of the total time between the reporting of the first and last seasonal isolates.2 The same is true in Wiscon­ sin, where we have prospectively tested patients in a population­based cohort over three seasons. Nichol et al. should consider additional analyses to determine whether the benefit of vaccination with respect to mortality varies over time within each influenza season. If the effect is real, the benefit should be greatest during the 4­to­8­week period of maximum virus circulation and should be much lower at the beginning and end of the season.