More than 1.1 million Americans signed up for an insurance plan through the federal health-care marketplace during its initial enrollment period, with more than 975,000 enrolling in December alone, the Obama administration announced Sunday.

The new figures, which came as the administration reworked its computer system to extend the deadline for an extra day, until midnight on Dec. 24, suggest that federal officials are making up some ground after glitches and processing errors made difficult to access and navigate during its first two months of operation.

So far, nearly 2 million Americans — who were either uninsured or had to change coverage after their existing plans were canceled — have signed up under the new health-care law on state and federal marketplaces. Roughly 850,000 people have enrolled through the state-run exchanges, according to Charles Gaba, a Web designer tracking enrollment numbers.

The administration is still far short of the enrollment targets it set just before the system was launched Oct. 1. The Department of Health and Human ­Services had anticipated that 3.3 million people would have signed up by now, according to a Sept. 5 agency memo.

Still, officials celebrated the end-of-year results.

“We are in the middle of a sustained, six-month open enrollment period that we expect to see enrollment ramp up over time, much like other historic implementation efforts we’ve seen in Massachusetts and Medicare Part D,” Centers for Medicare and Medicaid administrator Marilyn Tavenner wrote on the HHS blog, referring to the ­nation’s first health-insurance ­exchange under Massachusetts Gov. Mitt Romney (R), in 2006, and the prescription drug coverage expansion enacted during President George W. Bush’s administration. “In part, this was because we met our marks on improving the site supported 83,000 concurrent users on December 23rd alone.”

The next critical deadline for enrollment is March 31, after which individuals face a tax penalty if they remain uninsured.

Some experts on health-care policy say the December surge increases the possibility that the law could meet federal projections of 7 million enrollments by March 31, 2014.

“It is starting to track with what people, particularly the Congressional Budget Office, projected originally,” said Larry Levitt, senior vice president at the Kaiser Family Foundation. “December is the first month where federal sign-ups have kept up with state sign-ups, too.”

Senior administration officials have said they expected enrollment to start slowly after the Web site’s October launch and remain relatively low in November, before accelerating in December as many Americans sought out plans that would take effect Jan. 1.

That pattern materialized, although at a much lower level than officials initially expected. As of Dec. 22, 890,000 Americans had enrolled on the federal exchange, according to government figures that had not been made public, meaning that more than 200,000 people chose health plans on Dec. 23 or 24. By contrast, roughly 137,000 people signed up through the federal system in its first two months of operation.

Delayed by glitches

At least some of the late-
December shoppers didn’t mean to procrastinate, but technical issues thwarted their earlier attempts to sign up for coverage.

Anita Pinser, 62, tried to buy coverage Oct. 1 but finally enrolled in a plan Dec. 22.

“I kept trying and trying,” said Pinser, a former human resources professional who lives just outside Charlotte and who was laid off in 2008. “I called the 800 number and found out everyone was having problems.”

When she saw a posting on Twitter Dec. 22 about the impending deadline, she decided it was time to give it a final try. “I had my account set up, so all I really had to do was click on the coverage that I wanted,” she said. “It was really easy. The only bad experiences I had were the first month, when no one could sign up.”

But others are still experiencing serious problems with the Web site. David and Karen Hart live in Santa Rosa Beach, Fla., and both are self-employed. Their existing Blue Cross Blue Shield policy was canceled in the fall and then reinstated, but they picked a new plan on and paid for it on Dec. 19.

When he noticed that their payment hadn’t been deducted from their bank account, David Hart called Florida Blue on Dec. 27; he was told that the government’s computer system had canceled their plan. His wife spent more than four hours on a federal hotline Saturday, at which point she was told the insurer had jettisoned their policy.

“I’m a 56-year-old, and I have had health insurance my entire life. It’s just astounding that we’ve had to go through this,” David Hart, an artist, said in an interview. He noted that he and his wife support the new law but have been dismayed by how federal officials are implementing it. “We think it’s good policy, but what they’re doing is atrocious.”

By Sunday afternoon, CMS officials were investigating the Harts’ complaint.

America’s Health Insurance Plans spokesman Robert Zirkelbach, whose group represents insurers, said firms “are working around the clock to process the high volume of enrollments that they have received from the exchanges. While there are still some ongoing challenges with the back-end systems, including so-called ‘orphan records’ where the enrollment files are never received, health plans are working with CMS to resolve those issues as quickly as possible so that consumers’ coverage can begin in January.”

The CMS did not release any demographic information about who has enrolled, including the age breakdown. Those numbers are important, since insurers need a well-distributed risk pool in the exchanges to keep premium prices in check.

Outreach efforts

The administration is gearing up to preempt any health-care access problems that may crop up starting next month, publishing online guides on how to determine which doctor visits and prescription drugs are covered under the new system.

In a statement Sunday, the CVS pharmacy chain said that its employees “will provide information to help patients contact the appropriate health plan or insurance exchange,” and work with insurers and government agencies to “help minimize disruptions to care whenever possible.”

“In some circumstances and based on clinical considerations, we may also assist patients with a transitional supply of a prescription to a patient experiencing a temporary disruption in coverage to support their continuity of care,” CVS said.

Officials also plan to intensify their outreach to young adults, Latinos, African Americans, women and other key groups after Jan. 1, they said, through intermediaries including celebrities, local activists, provider networks, nurses, doctors, churches, mosques and synagogues.

“We are eager to assist millions more Americans gain the health security offered by the Affordable Care Act in the weeks and months ahead,” Tavenner wrote.

Former Vermont governor Howard Dean (D) said on “Fox News Sunday” that despite the problems of getting young people to sign up, he was confident that President Obama’s health-care law would be “running a lot more smoothly” by March.

The former Democratic National Committee chairman, who is also a doctor, accused critics of overstating the problems. “I think the first year is going to be more successful than most people think,” he said.

Emily Wax contributed to this report.