On the basis of early estimates of antibody positivity to pandemic infl uenza virus in the community, we assumed 20% infection rates in clinical staff and 10% rates in nonclinical staff. We calculated that 438 participants were required to achieve 80% power to detect this difference using a 0.05 two-tailed signifi cance level. The primary outcome was the presence of a positive antibody titer in the fi rst serum sample, indicating likely pandemic infl uenza infection.