Immunization is an essential component for reducing under-five mortality. Immunization coverage estimates are used to monitor coverage of immunization services and to guide disease eradication and elimination efforts. It is a good indicator of health system performance.

BCG immunization coverage among 1-year-olds (%) (WUENIC)
The percentage of live births who have received one dose of bacille Calmette-Guérin (BCG) vaccine in a given year . The indicator is estimated as the percentage of children ages 12–23 months who received one dose of the bacille Calmette-Guérin (BCG) vaccine before the survey.

Diphtheria tetanus toxoid and pertussis (DTP3) immunization coverage among 1-year-olds (%) (WUENIC)
The percentage of one-year-olds who have received three doses of the combined diphtheria, tetanus toxoid and pertussis containing vaccine in a given year. The indicator is estimated as the percentage of children ages 12–23 months who received three doses of the combined diphtheria, tetanus toxoid and pertussis vaccine time before the survey.

Hepatitis B (HepB3) immunization coverage among 1-year-olds (%) (WUENIC)
The percentage of one-year-olds who have received three doses of hepatitis B containing vaccine in a given year. The indicator is estimated as the percentage of children ages 12–23 months who received three doses of hepatitis B vaccine either any time before the survey.

Hib (Hib3) immunization coverage among 1-year-olds (%) (WUENIC)
The percentage of one-year-olds who have received three doses of the combined diphtheria, tetanus toxoid and pertussis containing vaccine in a given year. The indicator is estimated as the percentage of children ages 12–23 months who received three doses of Haemophilus influenzae type B vaccine any time before the survey.

Measles-containing-vaccine first-dose (MCV1) immunization coverage among 1-year-olds (%) (WUENIC)
The percentage of children under one year of age who have received at least one dose of measles-containing vaccine in a given year. For countries recommending the first dose of measles vaccine in children over 12 months of age, the indicator is calculated as the proportion of children less than 12-23 months of age receiving one dose of measles-containing vaccine.

Measles-containing-vaccine second-dose (MCV2) immunization coverage by the locally recommended age (%) (WUENIC)
The percentage of children who have received two doses of measles containing vaccine (MCV2) in a given year, according to the nationally recommended schedule. The indicator is estimated as percentage of children who received the 2nd dose of measles containing vaccine according to the nationally recommended schedule.

Polio (Pol3) immunization coverage among 1-year-olds (%) (WUENIC)
The percentage of one-year-olds who have received three doses of polio containing vaccine in a given year. The indicator is estimated as the percentage of children ages 12–23 months who received three doses of the polio vaccine before the survey.

Pneumococcal Conjugate vaccines (PCV3) immunization coverage among 1-year-olds (%) (WUENIC)
The percentage of one-year-olds who have received three doses of pneumococcal conjugate vaccine (PCV3) in a given year. The indicator is estimated percentage of surviving infants who received the 3rd dose of pneumococcal conjugate vaccine. In countries where the national schedule recommends two doses during infancy and a booster dose at 12 months or later based on the epidemiology of disease in the country, coverage estimates may reflect the percentage of surviving infants who received two doses of PCV prior to the 1st birthday.

Meningococcal A conjugate (MENGA) vaccine immunization coverage (%)
The percentage of children who received one dose of meningococcal A conjugate vaccine. MENGA coverage estimates produced for countries in the meningitis belt of sub-Saharan Africa.

Yellow fever vaccine (YFV) immunization coverage among 1-year olds (%)
The percentage of surviving infants who received one dose of yellow fever vaccine in countries where YFV is part of the national immunization schedule for children or is recommended in at risk areas; coverage estimates are annualized for the entire cohort of surviving infants.

Service/facility reporting system ("administrative data"): Reports of vaccinations performed by service providers (e.g. district health centers, vaccination teams, physicians) are used for estimates based on service/facility records. The estimate of immunization coverage is derived by dividing the total number of vaccinations given by the number of children in the target population, often based on census projections. Household surveys: Survey items correspond to children’s history in coverage surveys. The principal types of surveys are the Expanded Programme on Immunization (EPI) 30-cluster survey, the UNICEF Multiple Indicator Cluster Survey (MICS), and the Demographic and Health Survey (DHS).

Distinction is made between situations where data reported by national authorities accurately reflect immunization system performance and those where the data are likely compromised and may present a misleading view of immunization coverage. While there are frequently general trends in immunization coverage levels, no attempt is made to fit data points using smoothing techniques or time series methods. The estimates are informed and constrained by the following heuristics: Country–specific: Each country's data are reviewed individually; data and information are not "borrowed" from other countries. If national data are available from a single source, the estimates are based solely on that source, supplemented with linear interpolation to impute values for years where data are not available. If no data are available for the most recent estimation period, the estimate remains the same as the previous year's. If new data or information subsequently become available, the relevant portion of the time series is updated. Consistent trends and patterns: If survey data tend to confirm (e.g., within +/- 10% points) reported data, the estimates are based on reported data. If multiple survey points show a fairly consistent relationship with the trend in reported data and the survey data are significantly different from reported data, the estimates are based on reported data calibrated to the level established by the survey data. If survey data are inconsistent with reported data and the survey data appear more reliable, coverage estimates are based on survey data and interpolation between survey data points for intervening years. If multiple data points are available for a given country, vaccine/dose, and year data points are not averaged; rather potential biases in each of the sources are considered and an attempt to construct a consistent pattern over time, choosing data with the least potential for bias consistent with temporal trends and comparisons between vaccines is made. If coverage patterns are inconsistent between vaccines and dose number, an attempt to identify and adjust for possible biases is made. If inconsistent patterns are explained by programmatic (e.g., vaccine shortage) or contextual events (e.g., "international incidences") the estimates reflect the impact of these events. When faced with situations where several estimates are possible, alternative explanations that appear to cover the observed data are constructed and treated as competing hypotheses., local information is considered, potential biases in the data identified and the more likely hypothesis identified. Recall bias adjustment: In instances where estimates are based primarily on survey data and the proportion of vaccinations based on maternal recall is high, survey coverage levels are adjusted to compensate for maternal recall for multi-dose antigens (i.e., DTP, POL, HepB and Hib) by applying the dropout between the first and third doses observed in the documented data to the vaccination history reported by the child's caretaker. No coverage greater than 100%: Coverage levels in excess of 100% are occasionally reported. While such coverage levels are theoretically possible, they are more likely to be the results of systematic error in the ascertainment of the numerator or the denominator, a mid-year change in target age-groups, or inclusion of children outside the target age group in the numerator. The highest estimate of coverage is 99%. Local knowledge incorporated: By consulting local experts an attempt to put the data in a context of local events - those occurring in the immunization system (e.g. vaccine shortage for parts of the year, donor withdrawal, change in management or policies, etc.) as well as more widely-occurring events (e.g. international incidences, civil unrest, etc.) is made. Information on such events is used to support (or challenge) sudden changes in coverage levels. Description and dissemination of results: For each country, year and vaccine/dose the WHO and UNICEF estimates are presented in both graphic and tabular forms along with the data upon which they are based. The estimates are "thickened", by providing a description of the assumptions and decisions made in developing the specific estimates. Predominant type of statistics: unadjusted and adjusted

Global and regional coverage is a weighted sum of WHO/UNICEF estimates of national coverage by target population from the United Nations Population Division's World Population Prospects. The size of the target population is the national annual number of live births

Annual

The quality of the estimates are determined by the quality and availability of empirical data. Vaccination is relatively easy to measure and two methods - facility reports and surveys - have been developed, each of which, when properly designed and implemented, provides accurate and reliable direct measures of coverage levels. Implemented jointly , they provide a validation of coverage levels. However, both methods are subject to biases. In some instances, these biases may be identified and corrected and we have attempted to do so. These data are supplemented with local consultations that often explain inconsistencies and anomalies in the data and provide insight into forces that influence coverage levels. WHO and UNICEF are working closely with countries to improve the quality and usefulness of coverage monitoring data systems.

dumolardl@who.int

Laure Dumolard

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