Serological evidence indicates that measles virus (MV) could circulate in seropositive, fully protected populations. Among individuals fully protected against disease, those prone to asymptomatic secondary immune response are the most likely to support subclinical MV transmission. The serological characteristics of protected subjects who developed secondary immune response after reexposure to measles have been described recently [Huiss et al. (1997): Clinical and Experimental Immunology 109:416-420]. On the basis of these data, a threshold of susceptibility was defined to estimate frequencies of secondary immune response competence in different populations. Among measles, late convalescent adults (n = 277) and vaccinated high school children (n = 368), 3.2-3.9% and 22.2-33.2%, respectively, were considered susceptible to secondary immune response. A second vaccination did not seem to lower this incidence. Even when estimates of symptomatic secondary immune response (e.g., secondary vaccine failure) were taken into account, susceptibility to subclinical secondary immune response was still 5-8 times higher after vaccination than after natural infection. Although viral transmission between protected individuals has never been directly demonstrated, the data describe a population in which protected but infectious persons could potentially be of epidemiological importance.