Seroprevalence of Measles IgG Antibodies Among Vaccinated Children in Urban Settlements

Abstract

Measles vaccination programmes are usually judged by how many children have been vaccinated, but a vaccinated child is not necessarily a protected one, and surveys of measles IgG antibodies are the commonest way of asking the second question. This narrative review considers what such a seroprevalence figure can and cannot tell us when it is measured in vaccinated children living in urban settlements. It draws on published work on the virus and its immune effects, on antibody as a marker of protection, on the effect of age at vaccination and maternal antibody, on persistence and waning, and on population immunity in crowded urban settings, and it reports no new data. The review finds that an IgG result depends heavily on the assay used, that a negative or equivocal result in a vaccinated child has several distinct explanations of which true vaccine failure is only one, and that surveys with different methods and age groups cannot be compared at face value. Urban settlements add particular problems of denominators, documentation and mobility. We propose a short set of questions for appraising a survey and conclude that seroprevalence is best read as a measure of programme performance in a defined group, not as a verdict on the vaccine.