Obama administration officials on Thursday predicted health insurance premiums would be stable next year despite concerns that not enough young and healthy people signed up through the online insurance exchanges.

“The risk pool is fundamentally large and varied to support that kind of pricing…in every state,” said Mike Hash, director of the office of health reform for the Health and Human Services Department. “We believe … premiums will be stable.”

The upbeat assessment came as the administration offered the broadest glimpse yet of the demographics of the 8 million Americans who enrolled in health insurance through the health law’s online marketplaces through mid-April. Open enrollment for this year has ended in nearly all states and will re-open in November.

About 28 percent of enrollees nationwide were the coveted 18- to 34-year-olds, the group least likely to need health services. Analysts have debated whether enough young and healthy people have enrolled to ensure that premiums don’t spike in 2015 because of the significant number of older enrollees, many of them with expensive health issues. Insurance premiums for next year won’t be known until fall.

The report details the scope of the last-minute surge –about 900,000 people, or 12 percent of the more than 8 million enrollees, signed up after March 31.

Florida enrolled nearly 1 million people, far more than any federal exchange state — and about 250,000 more people than Texas, which has a larger population and more uninsured residents. Both Florida and Texas were among a handful of states whose enrollment doubled since March 1.

More than 4.8 million additional individuals enrolled in Medicaid and the Children’s Health Insurance Program through the end of March, compared to enrollment before the marketplace opened last October. About half the states expanded Medicaid under the law to cover everyone with incomes under 138 percent of the federal poverty level, or $15,800 for an individual.  Some of those new Medicaid enrollees were previously eligible but not enrolled in the program.

The report found that 63 percent of enrollees who answered questions about their racial or ethnic background through the federal exchange identified themselves as white, 11 percent as Latino and 17 percent as black. Almost one third of enrollees did not provide that information, according to the report released the Obama administration.

The largest number of Latinos enrolled in border states, including Texas at 33.6 percent, New Mexico at 31.1 percent, Arizona at 24 .2 percent and Florida at 19.2 percent.

The states participating in the federal exchange with the largest share of African-American enrollees were Mississippi at 59.5 percent, Georgia at 38.6 percent and Louisiana at 37.9 percent.

The highest enrollment of Asians among federal exchange states occurred in Virginia at 17.7 percent, New Jersey at 16.3 percent and Georgia at 14.8 percent.

Among the 8 million enrollees, 54 percent are women and 46 percent are men. Federal officials said they expected that gender gap because women are generally more likely to buy insurance than men.

About two-thirds of enrollees chose a mid-level, or silver plan—largely because that was the best deal for consumers getting a government subsidy. Overall, about 85 percent of people buying plans received subsidies.

Just 20 percent chose bronze plans, which had lower monthly premiums but higher deductibles and co-payments.

Nationally, about 60 percent of  13.5 million people who started to apply for coverage on the marketplaces–and were deemed eligible for the exchange plans based on their income and insurance states — completed the enrollment process.  About 63 percent of people completed the enrollment process on the state exchanges and 57 percent completed it on the federal exchange used by 36 states.

The 8 million-plus enrollees exceeded the projection made by the Congressional Budget Office, which had originally forecast 7 million would sign up in 2014, then downgraded that to 6 million after the rocky rollout of the federal exchange made it almost impossible for people to sign up in October and November.