WASHINGTON - The number of people signing up for health insurance through the federal marketplace soared in the final weeks before the enrollment deadline, exceeding the number who had signed up in the previous five months, President Barack Obama’s administration said Thursday. The late surge raised the total number of people enrolled through federal and state exchanges to more than 8 million.

For the first time, the administration provided some information about the racial and ethnic background of people signing up for insurance plans offered under the Patient Protection and Affordable Care Act.Of the 3.8 million people in the federal exchange who voluntarily disclosed such information, officials said, 63 percent were white, 17 percent were black, 11 percent were Hispanic and 8 percent were Asian.

Of those buying insurance in the exchanges, 2.2 million, or 28 percent, were young adults ages 18 to 34. Insurers covet people in that age bracket because they tend to be healthier and file fewer claims than older consumers.

The total number of people signing up through the federal exchange soared to more than 5.4 million over the final weeks, from 2.6 million at the end of February. And the total in state exchanges increased 59 percent, from 1.6 million to 2.6 million at the end of February.

The numbers reported Thursday include people selecting health plans through March 31, as well as people allowed to finish applications in the first 19 days of April because they had been stymied in earlier efforts to enroll.

In Arkansas, enrollment climbed to 43,446 as of April 19. That was an increase of more than 58 percent from the 27,395 people the Health and Human Services Department reported had enrolled as of March 1.

In addition, the Arkansas Department of Human Services has reported that 121,442 people who qualified for coverage under the state’s expanded Medicaid program were receiving coverage through exchange plans under Arkansas’ so-called private option as of March 31.

More than 16,000 others had been assigned to Arkansas’ traditional Medicaid program because their health needs were considered exceptional, and more than 17,000 had been approved for Medicaid coverage but had not completed enrollment.

The states with the largest numbers of people selecting plans through the exchanges during the six-month open-enrollment period included California (1.4 million), Florida (983,800), Texas (733,800), New York (370,500), North Carolina (357,600), Pennsylvania (318,100) and Georgia (316,500).

California and New York run their own exchanges, while the other five use the federal marketplace.

Despite the success of California and New York, enrollment lagged the most in states that were running their own marketplaces, including Massachusetts and Oregon, which struggled with their online systems. The two states fell more than 70 percent short of their goals.

Even several states considered successes, including Kentucky, had relatively few sign-ups for marketplace plans, though Kentucky had strong Medicaid enrollment. Altogether, 20 states fell short of projections made last year by the U.S. Department of Health and Human Services.

On Thursday, as in the past, federal officials said they did not know how many of the people signing up had paid their initial premiums, as required to activate their coverage. And officials said they did not know how many were previously uninsured.

A House committee said Wednesday that, based on information from insurers participating in the health exchange, only two-thirds of people had paid. Insurance industry officials previously have put the payment rate at about 85 percent.

Wellpoint, one of the nation’s largest insurers, reported this week that about 90 percent of its new customers had paid their premiums. The company also backed off earlier warnings about big 2015 premium increases, reporting in a call with investors that a late surge of young customers had improved the company’s risk pool.

Michael Hash, the director of the Office of Health Reform at the Health and Human Services Department, said he was confident that “premiums will be stable next year” because the pool of new policyholders was “sufficiently large and varied.”

But Larry Levitt, an insurance expert and senior vice president of the Kaiser Family Foundation, wasn’t as optimistic.

“While there is a lot of focus on the national numbers, it’s what is happening state by state that will affect how much premiums rise next year,” he said. “Insurers set premiums based on who signs up in a state. If lots of healthy people enrolled in California, that doesn’t spill over into Texas and help the risk pool there.”

Industry officials are most concerned about states where insurers continue to sell health plans that do not meet new standards set out in the law.These noncompliant plans are not being offered on the new marketplaces and allow many healthy consumers who have such plans to remain out of their state marketplaces.

That, in turn, means that the mix of consumers who are getting health plans on the state marketplaces tends to be sicker, creating a bad risk pool. The marketplaces depend on having a mix of healthy and unhealthy consumers to balance risk and keep premiums in check.

About half the states have allowed insurers to continue to sell these plans, an option Obama gave states last year after a public outcry over the cancellation of many plans.

Kathleen Sebelius, the outgoing secretary of health and human services, offered an upbeat interpretation of the numbers, which exceeded the administration’s original goal to sign up 7 million people in the health exchanges.

“In addition,” Sebelius said, “over 4.8 million more people have been covered by states through Medicaid” and the related Children’s Health Insurance Program. “Around 3 million more Americans under 26 are covered under their parents’ plans, and recent estimates show that an additional 5 million people have purchased coverage outside of the marketplace in Affordable Care Act-compliant plans.”

“Together,” she said, “we are ensuring that health coverage is more accessible than ever before, which is important for families, for businesses, and for the nation’s health and well-being.”

Enrollment got off to a slow start because of technology problems that crippled the federal exchange and hobbled some state-run exchanges after they opened Oct. 1.

In October, just 26,800 people signed up for coverage in the federal exchange, and 79,400 in state exchanges. The cumulative total climbed to 364,700 in November and 2.15 million in December, as people rushed to obtain coverage for 2014. The total then rose to 3.3 million in January and 4.2 million in February.

Obama announced April 17 that 8 million people had signed up, and he said that“Democrats should forcefully defend and be proud of” the achievement.

“I don’t think we should apologize for it, and I don’t think we should be defensive about it,” Obama said.

Some Democrats have been skittish on the matter, in the face of opinion polls indicating that many people have unfavorable views of the law, which was passed four years ago without any Republican votes.

Since the law was signed by Obama, Democrats had been predicting that public support for it would increase as people learned more about it and experienced its benefits.

The intense public focus on the need for health insurance also has led to an increase in enrollment in Medicaid, even in some states that did not expand eligibility.

For most Americans, the open-enrollment period for the purchase of private health plans on the exchanges ended March 31 and will reopen in November. By contrast, people can apply at any time for Medicaid, the federal-state program for low-income people.

Jason Furman, the chairman of the president’s Council of Economic Advisers, said Wednesday that an increase in national health spending in the first quarter of this year “appears to have been driven by greater use of health-care services by people who gained insurance” under the Affordable Care Act.

But he said that “this increase in utilization is neither a surprise nor a cause for concern,” and he noted that “health-care prices continued to increase exceptionally slowly.”

Information for this article was contributed by Robert Pear of The New York Times; by Noam Levey of the Tribune Washington Bureau; and by Andy Davis of the Arkansas Democrat-Gazette.

Front Section, Pages 1 on 05/02/2014