ESTIMATED SUSCEPTIBILITY TO ASYMPTOMATIC SECONDARY IMMUNE-RESPONSE AGAINST MEASLES IN LATE CONVALESCENT AND VACCINATED PERSONS

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 immu ne 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 descri bed recently [Huiss et al. (1997): Clinical and Experimental Immunolog y 109:416-420]. On the basis of these data, a threshold of susceptibil ity was defined to estimate frequencies of secondary immune response c ompetence in different populations. Among measles, late convalescent a dults (n = 277) and vaccinated high school children (n = 368), 3.2-3.9 % and 22.2-33.2%, respectively, were considered susceptible to seconda ry immune response. A second vaccination did not seem to lower this in cidence. Even when estimates of symptomatic secondary immune response (e.g., secondary vaccine failure) were taken into account, susceptibil ity to subclinical secondary immune response was still 5-8 times highe r after vaccination than after natural infection. Although viral trans mission between protected individuals has never been directly demonstr ated, the data describe a population in which protected but infectious persons could potentially be of epidemiological importance. J. Med. V irol. 56:85-90, 1998. (C) 1998 Wiley-Liss, Inc.