Characteristics of asymptomatic secondary immune responses to measles virus in late convalescent donors

Among 44 fully protected, late convalescent adults re-exposed to measles, four developed an asymptomatic secondary immune response (SIR) with a significant increase in measles virus (MV)-specific IgG and low IgM. The boosted antibodies were mainly of the IgG1 subclass and reacted with the nucleoprotein and the haemagglutinin protein. About 30 weeks after re- exposure, antibody levels had decreased by 35-50%, suggesting that the booster effect may only be transient. SIR was only found in individuals with a pre-exposure IgG level below a well defined threshold. Antibody levels above this threshold fully protected against SIR. SIR seems to be an 'all or none response' where the magnitude of increase in specific IgG is independent of pre-exposure antibody levels as long as these are below the above threshold. In combination with pro-exposure neutralizing and haemagglutination inhibiting titres, a threshold was defined below which SIR is likely to occur. This may be useful to predict susceptibility to SIR in a given population, since individuals undergoing clinically inapparent SIR are among seropositive subjects, the most likely candidates to support transmission of virus.