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Improvements needed for accurate vaccination coverage rates
Lessler J. PLoS Med. 2011;doi:10.1371/journal.pmed.1001110.
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Current measurements used to determine the success and rates of immunization may be flawed and inconsistent. However, results of a new study report that estimates of vaccination coverage may be improved by combining administrative data with survey data.
“Reliable estimates of vaccination coverage are key to managing population immunization status,” study researcher Justin T. Lessler, PhD, an assistant professor at Johns Hopkins Bloomberg School of Public Health, said in a press release. “Currently, the performance of routine and supplemental immunization activities is measured by the administrative method, which leads to coverage estimates that are often inconsistent with the proportion reporting vaccination in cross-sectional surveys. Furthermore, administrative coverage does not tell you how many people are systematically missed by vaccination activities. We estimated that the size of the population never reached by any activity was high in Sierra Leone and Madagascar, 31% and 21%, respectively. But it was much lower in Ghana, only 7%.”
Their estimates of routine coverage were, in general, lower than WHO and UNICEF estimates. The researchers developed a likelihood framework for estimating the effective coverage of vaccination programs using cross-sectional surveys of vaccine coverage combined with administrative data. The team applied its method to measles vaccination in Ghana, Madagascar and Sierra Leone by using data from each country’s most recent Demographic and Health Surveys and administrative coverage data reported to WHO.
The team estimated that 93% (95% CI, 91-94) of the population in Ghana was ever covered by any measles vaccination activity. In Madagascar, it was 77% (95% CI, 78-81), and 69% (95% CI, 67-70) in Sierra Leone.
According to the findings, “within-activity” inefficiencies were estimated to be low in Ghana and higher in Sierra Leone and Madagascar. The model successfully fits age-specific vaccination coverage levels seen in Demographic and Health Surveys data, which differ markedly from those predicted by naive extrapolation from country-reported and WHO-adjusted vaccination coverage.
The administrative method divides the number of doses distributed by the size of the target population. The investigators developed a method for estimating the effective coverage of vaccination programs using cross-sectional surveys of vaccine coverage combined with administrative data.
“This method not only attempts to correct coverage estimates, but also distinguishes between issues of overall coverage and vaccine within activity inefficiencies. For our technique to be useful, countries must have cross-sectional data on vaccine coverage for children across a range of ages, some of an age where they have been exposed to multiple vaccination activities,” study researcher Derek A. Cummings, PhD, assistant professor in the department of epidemiology, Johns Hopkins Bloomberg School of Public Health, said in a press release.
Disclosure: The study was supported by grants from the Vaccine Modeling Initiative of the Bill & Melinda Gates Foundation, the US Department of Homeland Security, the NIH, the Burroughs Wellcome Fund and the Royal Society.
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Accurately determining immunization coverage is critical to assessing immunization program performance and identifying problems in immunization levels that require attention. In the developing world, coverage is most often measured using the “administrative method,” which consists of collecting the number of doses administered in given clinics that serve a population, and then dividing that number by the estimated target population. This method, although easy to implement, is often inaccurate, often leading to falsely high assessments of coverage. Lessler and colleagues have developed a method to use the administrative data along with data from a representative randomized survey, conducted infrequently, to more accurately estimate coverage. They model inefficiency (doses given to children who do not need them), the proportion of children who are accessible to health services and other terms. This method is promising but will need further evaluation to determine how accurately it estimates true immunization coverage. In particular, they use Demographic Health Surveys, which rely on either immunization record cards or parental recall as the “gold standard,” and the validity of that standard should be evaluated further.
– Walter A. Orenstein, MD
Infectious Diseases in Children Editorial Board
– Matthew Behrend, PhD
Intellectual Ventures Lab
Bellevue, Washington
Disclosures: Drs. Orenstein and Behrend report no relevant financial disclosures.
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