Avoiding the Will O’ the Wisp: Challenges in Measuring High Levels of Immunization Coverage with Precision

There have been tremendous improvements in immunization coverage since the Expanded Programme on Immunization was launched. We highlight 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. In fact, when comparing across groups at high levels of coverage, error in target population estimates can obscure differences in immunization coverage.