The number of young adults in the U.S. lacking medical coverage has shrunk by 2.5 million since the new health care overhaul law took effect, according to a new analysis the Obama administration is to release Wednesday.

That drop is 2 1/2 times as large as the one indicated by previous government and private estimates from earlier this year, which showed about 1 million Americans ages 19-25 had gained coverage.

Under the health overhaul, children can remain on their parents' health insurance plans until they turn 26, and families have flocked to sign up young adults making the transition to work in a challenging economic environment. But the fate of President Barack Obama's signature domestic accomplishment remains uncertain, with the Supreme Court scheduled to hear a constitutional challenge next year, and Republican presidential candidates vowing to repeal it.

"The increase in coverage among 19- to 25-year-olds can be directly attributed to the Affordable Care Act's new dependent coverage provision," said a draft report from the Health and Human Services Department. "Initial gains from this policy have continued to grow as ... students graduate from high school and college." A copy of the report was obtained by The Associated Press.

HHS Secretary Kathleen Sebelius is scheduled to release the findings Wednesday.

The health care law's main push to cover the uninsured doesn't come until 2014. But the young adults' provision took effect last fall. Most workplace health plans started carrying it out Jan. 1.

Traditionally, young adults were more likely to be uninsured than any other age group.

Some are making the switch from school to work. Others are holding down low-wage jobs that don't usually come with health care. And some _ termed the "invincibles" _ pass up job-based health insurance because they don't think they'll use it and would rather get extra money in their paychecks.

Using unpublished quarterly statistics from the government's ongoing National Health Interview Survey, analysts in Sebelius' policy office determined that nearly 36 percent of those age 19-25 were uninsured in the third calendar quarter of 2010, before the law's provision took effect.

That translates to more than 10.5 million people.

By the second calendar quarter of 2011, the proportion of uninsured young adults had dropped to a little over 27 percent, or about 8 million people.

The difference _ nearly 2.5 million getting coverage _ can only be the result of the health care law, administration officials said, because the number covered by public programs like Medicaid went down slightly.

Overall, nearly 30 million Americans are between the ages of 19 to 25. For those who are little older, ages 26-35, the uninsured rate went up during the same period.