Influenza epidemic among a community of elderly people in spite of vaccination

An outbreak of influenza occurred in a nursing home of 81 vaccinated elderly people. The clinical attack rate was 73%. The responsibility of an A/H3N2 strain was proved in 12 patients and 2 staff members of the 15 investigated persons. The vaccine was effective in reducing mortality but not morbidity. An antigenic drift between the wild and the vaccine strains was involved in the inefficiency of the vaccination.