Of 51 countries that have received reward payments since 1999, six
overestimated their immunization gains by a factor of four, 10
overestimated them by a factor of two, and 23 by less than two. Eight
underestimated their progress.
Here is the article, interesting throughout. The bottom line is this:
Since 1986, progress in childhood immunization in the developing world
has been about half that officially reported by governments in the
developing world. Not only are year-to-year improvements overstated,
but the total percentage of children immunized is far lower than
publicly acknowledged, the study found.