- Findings concerning influenza vaccine efficacy in young, healthy adults are inconsistent. A high incidence of influenza in the winter of 1995 provided an opportunity to study the efficacy of influenza vaccine among young, healthy military personnel. Influenza activity was confirmed by isolation of influenza A and B viruses from nasopharyngeal swab specimens from hospitalized soldiers. Selfadministered questionnaires concerning vaccination status and disease symptoms were used in two study groups: recruits and veteran soldiers serving in different camps. Six hundred eighty-four individuals had received influenza vaccine and 652 had not. Vaccine efficacy was found to be 38.1% (P = .002) for preventing febrile illness with or without symptoms and slightly higher (41.6%; P < .001) for preventing fever together with upper respiratory tract symptoms. The current influenza vaccine significantly reduced febrile illness among healthy military personnel.