Comparative Estimates of Crude and Effective Coverage of Measles Immunization in Low-Resource Settings: Findings from Salud Mesoamérica 2015.

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

Proportion of card-positive children lacking antibodies in Nicaragua.

<p>This figure displays the proportion of children who lack measles antibodies across municipalities in Nicaragua, among children with health card documentation of receiving measles vaccination. In both countries, this proportion is very high in small number of municipalities and low in the remaining municipalities. The number in each municipality indicates sample size. Estimates are survey weighted.</p

Proportion of card-positive children lacking antibodies in Mexico.

<p>This figure displays the proportion of children who lack measles antibodies across municipalities in Mexico, among children with health card documentation of receiving measles vaccination. In both countries, this proportion is very high in small number of municipalities and low in the remaining municipalities. The number in each municipality indicates sample size. Estimates are survey weighted.</p

Crude and effective coverage of measles immunization by municipality in Nicaragua.

<p>This figure displays the crude (left) and effective (right) coverage of measles immunization across municipalities in Nicaragua. Crude coverage tends to be higher than effective coverage, but geographic patterns in high-performing and low-performing areas are similar. The number in each municipality indicates sample size. Estimates are survey weighted.</p

Crude and effective coverage of measles immunization in Nicaragua.

<p>This figure displays estimates of measles immunization coverage based on (1) the combination of child health cards and caregiver recall, (2) caregiver recall only, (3) child health card only, and (4) analyses of dried blood spot samples. Coverage estimates are highest according to the combination of child health card and caregiver recall, and lowest according to dried blood spot samples, and these differences were statistically significant. The figure restricted to the 299 children with coverage information from all three sources, excluding children with DBS collection within 28 days of vaccination. Lines indicate 95% confidence intervals. Estimates are survey weighted.</p

Crude and effective coverage of measles immunization in Mexico.

<p>This figure displays estimates of measles immunization coverage based on (1) the combination of child health cards and caregiver recall, (2) caregiver recall only, (3) child health card only, and (4) analyses of dried blood spot samples. Coverage estimates are highest according to the combination of child health card and caregiver recall, and lowest according to dried blood spot samples, and these differences were statistically significant. The figure is restricted to the 552 children with coverage information from all three sources, excluding children with DBS collection within 28 days of vaccination. Lines indicate 95% confidence intervals. Estimates are survey weighted.</p

Crude and effective coverage of measles immunization by municipality in Mexico.

<p>This figure displays the crude (left) and effective (right) coverage of measles immunization across municipalities in Mexico. Crude coverage tends to be higher than effective coverage, but geographic patterns in high-performing and low-performing areas are similar. The number in each municipality indicates sample size. Estimates are survey weighted.</p

Proportions of children with measles immunization information from caregiver recall, health cards, and dried blood spots.

<p>This figure displays the proportion of surveyed children aged 12 to 23 months in Mexico and Nicaragua with measles immunization information from survey and biomarker sources. Many caregivers struggled to recall the type and number of vaccines given, but we were able to collect health card documentation and DBS from a majority of children in both countries. This study focuses primarily on children with both health card and DBS sources, comprising 62.6% of the sample in Mexico and 67.1% of the sample in Nicaragua.</p