How Many Injections of Poliomyelitis Vaccine for Effective and Durable Immunity?

The degree and duration of the effects produced by three injections of killed-polio-virus vaccine were studied in a number of vaccinated children. The criterion used for determining immunity was the titer of antibody found in the blood at intervals up to four years after vaccination. The data, whether plotted singly for individual patients or collectively for groups, showed the initial rise following the vaccination, a secondary rise to a maximum after booster injections, and gradual decline from the maximum to a plateau. The evidence showed that the attainment of an effective level of immunity as a result of the three initial injections depended on the administration of a sufficient quantity of antigen and that this also determined the vigor of responses to subsequent booster injections. There is no evidence of harmful effect from as many as six injections, but there are not sufficient epidemiological reasons on which to base a campaign for a fourth injection. Multiple injections of a vaccine of low potency are not as reliable a way to induce immunity as are fewer injections of a vaccine of adequate potency.