Population immunity to measles and rubella viruses in rural Zambia

Background: Despite widespread use of measles vaccine, measles remains an important cause of child morbidity and mortality in sub-Saharan Africa. While rubella infection is typically mild, congenital rubella syndrome (CRS) is of concern for susceptible pregnant women. In contrast to measles, rubella and CRS epidemiology is poorly understood. A region-wide measles elimination goal is targeted for 2020 and the Global Alliance for Vaccines and Immunizations-funded combined measles-rubella vaccine will be introduced in the coming years. Gaps in the serosurvey literature for measles and rubella regarding immunity across the entire age spectrum, among males and females, and in rural settings must be addressed. Methods: We examined the age structure and spatial characteristics of persons susceptible and immune to measles and rubella viruses in rural Choma District, Southern Province, Zambia in 2008 and 2009. Dried blood spots were analyzed for the presence of measles and rubella immunoglobulin G antibodies. A distance decay effect, examining the association between distance from a residence to the nearest health center and measles serostatus, was evaluated. We estimated rubella and CRS incidence with a catalytic compartmental model. We separately modeled rubella incidence following the World Health Organization (WHO) vaccine introduction strategy of a Supplementary Immunization Activity (SIA) among persons aged 1 to 14 years old followed by routine vaccination of 1 year olds, with an age-structured Susceptible-Exposed-Immune-Recovered compartmental model. Results: Overall measles and rubella seropositivity was 68% and 50% respectively among the 632 study participants. Rubella seropositivity among women of childbearing age was 77%. Spatial variation in risk was observed for both measles and rubella susceptibles compared to immunes. Significant clustering of measles susceptibles, but not rubella susceptibles, was identified. We did not observe a distance decay effect. An estimated 519 rubella infections per 100,000 pregnancies and 133 CRS cases per 100,000 live births were predicted annually. Vaccine introduction may control rubella transmission among adults if 80% effective coverage is achieved. Conclusions: Measles and rubella population immunity was lower than that found in recent serosurveys. Older children and adolescents may need additional targeted measles vaccine activities to increase population immunity and achieve elimination. The WHO-recommended rubella vaccine introduction strategy seems appropriate for this setting.