The boys of Christ's Hospital experienced outbreaks of influenza A in 1972 (A/England/42/72), in 1974 (A/Port Chalmers), and in 1976 (A/Victoria). In each outbreak, the protective effect of inactivated influenza-A vaccine was limited to those boys, not already immune, who were vaccinated for the first time with the most up-to-date strain. Revaccination with the same strain did not increase the degree of protection, and revaccination with a later strain did not afford protection against subsequent challenge. The cummulative attack-rate in the three outbreaks was similar in all groups irrespective of vaccination history. These observations suggest that annual revaccination with inactivated influenza-A vaccine confers no long-term advantage.