Flu: effect of vaccine in elderly care home residents: a randomized trial.

OBJECTIVES

To determine whether assessing seroprotection after influenza vaccine and administering booster vaccination where not achieved reduces hospitalization and death. To estimate the overall seroprotection rate of influenza vaccine.

DESIGN

A two-arm, partially blind, randomized, multicenter, parallel-group, controlled trial.

SETTING

Twenty-six care homes in three South London boroughs in fall 2004.

PARTICIPANTS

Two hundred seventy-seven elderly permanent care home residents meeting eligibility criteria.

INTERVENTION: Postvaccination blood samples were randomized to booster evaluation or no booster evaluation (control). If evaluation revealed inadequate seroprotection, a booster vaccine was administered.

MEASUREMENTS: Primary outcome was hospitalization to end April 2005; secondary outcomes were death, antibiotic use, and seroprotection.

RESULTS

Sixty percent of the controls and 41% of the booster evaluation group responded to routine vaccination. Booster vaccination where indicated increased seroprotection rates in the booster evaluation group to 66%. Treatment groups did not differ in any outcome measures in the intention-to-treat analysis (hospitalization odds ratio=1.02, 95% confidence interval=0.55-1.87). There was a tendency towards greater differences between groups in the per-protocol analysis than in the intention-to-treat analysis, particularly regarding seroprotection rates. The same effect was observed in the a priori exploratory analysis of residents not seroprotected after routine vaccination alone.

CONCLUSION

In a year without circulating influenza, there is no clinical benefit of administering a booster vaccine if routine trivalent vaccination fails to result in seroprotection. Hemagglutination titers rose in two strains postbooster vaccination but fell against the novel strain, Wyoming. The benefit of such a booster strategy when influenza is prevalent thus remains uncertain.