RESUMEN
This report describes a follow-up serological study of 79 Brazilian children who, because of their young age, had failed to develop protective levels of immunity after vaccination against measles. There was serological evidence that infection with wild virus had occurred at a rate of about 17% per annum. Approximately 1(1/2) years after the initial vaccination, 46% of the uninfected children maintained very low levels of neutralizing antibody, but did not have a measurable haemagglutination-inhibition titre. Revaccination did not elicit an IgM response in most children, but stimulated anti-measles IgG production in all of them. In 36% of the children, the IgG titres fell again within three months to levels that may permit reinfection. If it is assumed that some of the persistent titres can be attributed to wild virus infection, the actual effect of revaccination would have been to immunize no more than 60% of the susceptible group. The results suggest that early administration of measles vaccine may produce a cohort of children with inadequate immunity who cannot be fully immunized by revaccination. The implications of these findings for measles immunization programmes are discussed.