But the enrollment numbers — released in a government report Monday — lagged behind the Obama administration's target of 3.3 million sign-ups by the end of December.
With nearly 500,000 enrollees through Dec. 28, California, which runs its own health insurance marketplace, continues to top all other states by a wide margin.
The national data also suggest that the marketplaces are still far more popular among older consumers, a trend that threatens to push up premiums unless more young, healthy Americans sign up for insurance.
A third of the new enrollees in October, November and December were between ages 55 and 64, according to the report from the federal Department of Health and Human Services. By contrast, only about a quarter were between ages 18 and 34 — well below the 40% goal that administration officials set. In California, the mix of enrollees by age was similar to the nationwide tally.
Insurance industry officials have been closely watching the mix of customers. Last week, insurance giant
However, Robert Zirkelbach, a spokesman for America's Health Insurance Plans, the industry's Washington-based lobbying arm, cautioned that it is premature to draw conclusions about the mix since the enrollment period lasts until the end of March.
"It is important to look at the enrollment picture over the whole six-month period," Zirkelbach said.
Obama administration officials expressed optimism Monday that the mix of customers will change.
"We think more and more young people will sign up," said Gary Cohen, who is overseeing the new marketplaces at the Centers for Medicare and Medicaid Services.
That prediction seems realistic to Larry Levitt, senior vice president at the nonprofit Kaiser Family Foundation, who studies insurance markets.
"I would expect enrollment to surge in second half, particularly in March," he said.
The state-based marketplaces — a centerpiece of the Affordable Care Act — were created so Americans who do not get coverage at work could select among plans that offer at least a basic set of benefits and cannot turn away sick people.
The marketplaces opened Oct. 1. Americans who make less than four times the federal poverty level — or about $94,000 for a family of four — qualify for government subsidies to offset the cost of their premiums.
To date, nearly 80% of new enrollees have qualified for financial assistance, according to the report.
Coverage started Jan. 1 for people who selected plans before Christmas. But Americans have until the end of March to choose a plan before they risk having to pay a fine for not having coverage.
Obama administration officials had hoped to get about 7 million people to sign up for plans in 2014, the first year of coverage under the law.
It remains unclear if that number is within reach. Problems with the federally run marketplaces in 36 states and the HealthCare.gov Web portal made it nearly impossible to shop for plans in these states in October and November.
The problems appear to have been largely fixed by December. More than 1 million people signed up for plans in December through federally run marketplaces, seven times the number in October and November.
There were fewer problems in the 14 states including California that operate their own insurance marketplaces.
Enrollment in these states also surged in December, with 729,000 new enrollees in the month, three times the total in the previous two months. In California, the report said, the state has signed up 498,794 people in private health plans since Oct. 1.
Florida, which relies on the federal government to run its marketplace, is now second in enrollments with 158,000. New York, which operates its own marketplace, is third.
The report put the number of new enrollees nationwide through December at 2.15 million, but officials have acknowledged that some of them may be counted more than once.
The Obama administration has not said how many of the new enrollees have paid their first premiums, a key indicator of how many people will actually have health coverage.
Many insurers have allowed consumers to pay their first premiums in January to accommodate those who may have been confounded by the website problems.
Still more consumers may be considering their options. About 3 million people have not yet selected a plan, but have filled out applications for coverage on the new insurance marketplaces since Oct. 1 and have been deemed eligible to get a health plan, according to the report.
The Obama administration did not release any data about how many of the new enrollees were previously uninsured, making it impossible to calculate how much the health law is actually expanding coverage.
Many Americans who had bought their own health insurance because they did not get coverage at work received cancellation notices last year because their plans did not meet new standards in the health law.
It remains unclear how many of those people have received new plans directly from their insurer, how many are shopping on the new marketplaces and how many are now uninsured.