By Julie Steenhuysen

CHICAGO, Dec 11 (Reuters) - Global childhood immunizations are growing at only about half the rate reported to global health agencies as countries receiving aid exaggerate coverage to meet performance goals, U.S. researchers said on Thursday.

They analyzed independent surveys and found gaps between actual rates of childhood immunization and estimates reported to the World Health Organization and the United Nations Children's Fund.

"An incentive to over-report progress, either intentionally or unintentionally, will always exist with performance-based payments," said a statement from Dr. Christopher Murray of the University of Washington in Seattle, whose study appears in the journal Lancet.

Murray and colleagues studied the number of children receiving the three-dose diphtheria, tetanus and pertussis (DPT3) vaccines in countries receiving aid money from the Global Alliance on Vaccines and Immunizations, or GAVI, a Geneva-based body working to improve access to vaccines.

They looked at 193 countries between 1996 and 2006 and found that since 1999, when GAVI was launched, officially reported estimates show a 9 percent jump in DPT3 vaccination coverage, while independent surveys showed only a 4.9 percent increase in global coverage.

They also found that the GAVI immunization services support program that pays countries $20 dollars for each additional child immunized leads to overreporting in two-thirds of the countries studied.

They said the gap between country-reported and independently reported data was especially wide in the Democratic Republic of the Congo, Guinea, Liberia, Mali, Niger and Pakistan.

"We don't know exactly why there is such a striking gap between the survey data and the country-reported data when it comes to over-reporting -- or in some cases under-reporting -- the number of additional immunizations," Stephen Lim, who worked on the study, said in a statement.

"What we do know is that there is a clear correlation between when those gaps start to widen and when GAVI started funding these countries."

The researchers called for independent measurement of immunization levels, which they said should be a condition of funding.

But Dr. David Bishai of Johns Hopkins School of Public Health in Baltimore, who wrote a commentary in the journal, said the findings were suggestive but not quite conclusive.

"I really want to caution readers to slow down and hold off in reaching the conclusion that, 'Aha! We caught them over-reporting,'" Bishai said in a telephone interview.

In his commentary, Bishai said the findings were tentative and voiced concern they may spark an "inquisition that diverts country vaccine staff away from the important job of immunizing children."

(Editing by Will Dunham and Bill Trott)

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