Avoiding the Will O' the Wisp: Challenges in measuring high levels of immunization coverage with precision

When the Expanded Programme on Immunization was launched in 1974, less than 5% of the world’s children were immunized during their first year of life against polio, diphtheria, tuberculosis, pertussis, measles and tetanus. Since that time, the successes of the Expanded Programme on Immunization have led to tremendous improvements in immunization coverage. Below we highlight that, even in the presence of perfect recording and monitoring of the number of children vaccinated, there are inherent challenges in measuring immunization coverage with precision as coverage levels increase due to the sensitivity of coverage to the accuracy of target population estimates.