Herd Immunity Following Subunit Influenza Vaccine Administration

An outbreak of Hong Kong influenza occurred in 1969 among highly susceptible communities in the Northern Territory of Australia. The epidemic commenced before the completion of a campaign to immunize as many people as possible, and especially those considered to be most at risk, with a desoxycholate-split influenza virus vaccine Seventeen of 20 communities which had not been given the vaccine before the disease first appeared suffered widespread and severe influenza with 12 deaths. No cases occurred at one locality where 84% of the population had been vaccinated, and there were mild outbreaks in only four of 29 part-immunized communities. In these four communities there was a much lower incidence of clinical influenza in the immunized than in the unimmunized. Vaccination of persons at greatest risk in the 29 communities limited the spread of the disease in the Territory.