Abstract
Timely and accurate measurement of population protection against measles is critical for decision-making and prevention of outbreaks. However, little is known about how survey-based estimates of immunization (crude coverage) compare to the seroprevalence of antibodies (effective coverage), particularly in low-resource settings. In poor areas of Mexico and Nicaragua, we used household surveys to gather information on measles immunization from child health cards and caregiver recall. We also collected dried blood spots (DBS) from children aged 12 to 23 months to compare crude and effective coverage of measles immunization. We used survey-weighted logistic regression to identify individual, maternal, household, community, and health facility characteristics that predict gaps between crude coverage and effective coverage. We found that crude coverage was significantly higher than effective coverage (83% versus 68% in Mexico; 85% versus 50% in Nicaragua). A large proportion of children (19% in Mexico; 43% in Nicaragua) had health card documentation of measles immunization but lacked antibodies. These discrepancies varied from 0% to 100% across municipalities in each country. In multivariate analyses, card-positive children in Mexico were more likely to lack antibodies if they resided in urban areas or the jurisdiction of De Los Llanos. In contrast, card-positive children in Nicaragua were more likely to lack antibodies if they resided in rural areas or the North Atlantic region, had low weight-for-age, or attended health facilities with a greater number of refrigerators. Findings highlight that reliance on child health cards to measure population protection against measles is unwise. We call for the evaluation of immunization programs using serological methods, especially in poor areas where the cold chain is likely to be compromised. Identification of within-country variation in effective coverage of measles immunization will allow researchers and public health professionals to address challenges in current immunization programs.
Highlights
Measles is an infectious vaccine-preventable disease that causes more than 125,000 worldwide deaths annually, most in children under 5 years of age [1]
Endemic measles transmission was first interrupted in Mexico in 1997 and there have been no confirmed cases in Nicaragua since 1995, both countries remain vulnerable to imported cases and outbreaks, in population clusters with low vaccination coverage [2]
Lapses in coverage in poor, rural areas of Mexico led to a devastating epidemic in 1989 and to reintroduction of endemic transmission in April 2000 [2]
Summary
Measles is an infectious vaccine-preventable disease that causes more than 125,000 worldwide deaths annually, most in children under 5 years of age [1]. Accurate measurement of immunization coverage is critical to preventing future outbreaks; concerns have been raised about the accuracy of current metrics [3,4,5] In both countries, existing data on vaccination coverage comes from national health surveys [6,7,8,9,10,11], which typically capture data from child health cards and rely on caregiver recall when cards are unavailable. Administrative estimates are subject to error and bias from both numerator data (number of doses distributed) and denominator data (the number of persons who should have received the vaccine) Most importantly, these sources do not capture the gap between crude (vaccination) and effective coverage (seroconversion) [13]
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