Administrative estimate of vaccine coverage
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Virtually all vaccination programmes, both routine and campaign, collect data from clinics and vaccination teams on how many people have been vaccinated. If one divides the total of these counts by the number of eligible people in the population, this produces an estimate of vaccine coverage.
If the vaccine (such as polio, diphtheria, pertussis, and tetanus vaccines) requires a series of doses to be effective, and the purpose of the coverage estimate is to determine what proportion of the target population is protected, the administrative coverage estimate should include only those children who have completed the entire series. In this case, personnel who do the vaccination must record which dose of vaccine in the series each child gets. Teams must then report the number of doses of vaccine administered according to the number in the series. For example, some children will receive their first dose of DPT, others will receive their second dose, and still others will receive their third and final dose. An administrative estimate of vaccine coverage for a multi-dose vaccine may include only those children who have received the final dose.
This is the most common method of estimating vaccination because the programme already collects the data to count how many people have received the vaccine. In addition, it is relatively easy to divide this by the estimated number of eligible people.
Measles vaccine provides full protection after one dose. A measles vaccination campaign in a refugee camp targeted children 6 months to 15 years without regard to prior vaccination status or other factors. Vaccination teams reported vaccinating 24,000 children. A recent camp registration counted 30,000 children in this age range. What is the administrative estimate of the coverage achieved by this measles vaccination campaign?
The administrative coverage can, and should, be estimated frequently using routine program data. This will provide up-to-date evaluations of the performance of the vaccination program
However, there are some problems with the administrative method:
Inaccurate numerator - The count of people vaccinated may be wrong
Survey teams may have incorrectly counted or logged the number of people vaccinated or the number of vaccine doses given. They may have included wasted doses in their count of the number of doses used. If large, multi-dose vials of vaccine are used, many doses may be discarded at the end of the day. If these doses are included in the count, the number of persons vaccinated is artificially inflated.
Team members, or anyone at higher levels, may exaggerate the number of people vaccinated to make their team look busy or to make the program look good. If team members are paid by the person vaccinated or receive a bonus for vaccinating more than a set number of people, this provides a strong incentive for such intentional distortion.
Because data on the number of people vaccinated must be added together at each level, any arithmetic error anywhere along the line may result in an inaccurate numerator.
Inaccurate denominator - The estimate of the size of the target population may be wrong
The estimate of population size is often very rough in humanitarian emergencies, especially early on during the acute phase when many emergency vaccination campaigns are done. Moreover, although one can estimate what percent of the population is less than 5 years of age from data on recent fertility, such estimates are not easily done for older children or subgroups of adults.
In addition, even if a registration of the population is done, ages may not be collected on each person or ages may be inaccurate, leading to an inaccurate estimate of the number of persons in a specific age group.
Reported administrative coverage estimates are routinely greater than 100%, an absurdity which demonstrates the inaccuracy of the numerator or denominator data used to calculate these estimate.