Immunization against childhood diseases such as diphtheria, pertussis, tetanus, polio and measles is one of the most important means of preventing childhood morbidity and mortality. Despite the low cost of basic childhood immunizations, nearly 3 million children still die each year from vaccine-preventable diseases. Achieving and maintaining high levels of immunization coverage must therefore be a priority for all health systems. In order to monitor progess in achieving this objective, immunization coverage data can serve as an indicator of a health system's capacity to deliver essential services to the most vulnerable members of a population.

This note discusses the use of trends in immunization coverage data, and argues that these be used as a proxy for monitoring overall health sector performance:
· Immunization is a health output with a strong impact on child morbidity, child mortality and permanent disability. The usefulness of immunization coverage is not simply as a measure of the implementation of one health intervention, but as a proxy for the overall performance of the health system to support priority health interventions.
· The target group consists of zero- to one year old children, and the members of the group consist of the cohort of children born each year. Immunization coverage is therefore a sensitive indicator: if measured annually, it can provide timely evidence of improvement and deterioration in current services.
· Measurement of immunization coverage can be relatively straightforward and inexpensive, and results in valid and verifiable information.
· Definitions used in surveys and health information systems to measure immunization coverage can be precise and objective, enabling comparisons across countries and over time.
· Immunization against a number of childhood diseases is a universally recommended, cost-effective public health priority, for which internationally adopted targets exist.
· Immunization coverage rates are frequently available at the subnational level, including at the district level. As health reform frequently includes decentralization, this is an important advantage for monitoring of impact and for targetting of service delivery.
· Immunization coverage rates are useful to monitor progress in expanding essential health services in adverse health settings, and as "safeguard" indicators when health system reforms are changing delivery or financing of health services in settings in which immunization coverage has already achieved high.

But not all immunization coverage data are reliable or precise, and comparability over time is sometimes limited. This note discusses measurement and interpretation issues for coverage data collected through surveys and administrative records.

The note includes an Annex with detailed reference tables showing time-series data of immunization coverage from 1980 to 1998 compiled by the World Health Organization for all countries, as well as the results of 64 Demographic and Health Surveys conducted between 1986 and 1998.