WASHINGTON — About 12.7 million Americans signed up for 2016 health insurance coverage through the government insurance exchanges, surpassing expectations, Health and Human Services Secretary Sylvia Burwell said earlier this month.

The government began offering subsidies for individual insurance in 2014 under the Affordable Care Act, frequently referred to as Obamacare, and charges a penalty to Americans who lack health insurance.

In 37 states, customers can buy these plans on HealthCare.gov, the federally run website, while the other states and Washington, D.C., run their own online exchanges.

Insurers say they have been struggling to make money on the health insurance exchanges, where low enrollment has contributed to high per-customer overhead and has made it a riskier business for them. Medical costs have also been an issue for insurers in 2015, with many reporting that they have booked unsustainable losses on these products.

In all, about 2.7 million people ages 18 to 34 signed up for the insurance, said Burwell. Andy Slavitt, who heads the Centers for Medicaid and Medicare Services division of the health department, said that the enrollment numbers exceeded the midpoint of its projection to have between 11 million and 14.1 million people signed up for 2016 health coverage at this point in the year.

“Open enrollment for 2016 is over (effective January 31), and we are happy to report it was a success,” said Burwell. “The Health Insurance Marketplace is changing people’s lives for the better.”

Burwell identified several ways in which the Marketplace exceeded expectations: 4 million new people enrolled in coverage in HealthCare.gov states; of the 9.6 million consumers who got coverage through HealthCare.gov, about 42% were new to the Marketplace in 2016; about 60% (2.4 million) of new enrollees in HealthCare.gov states signed up for January 1 coverage compared to about 40% (1.9 million) of new enrollees last year.

“Instead of waiting until the last moment, as we saw in previous years, people signed up for coverage by the first deadline because they wanted coverage to start as soon as possible,” pointed out Burwell. “About seven in 10 consumers with 2015 coverage came back to HealthCare.gov and actively selected a plan for 2016. Last year, about half of returning consumers actively selected a plan.

“More than 3.6 million people used the total cost calculator, provider or drug look up tools — yet another sign of the seriousness and time they put into their decisions — and a sign that these tools were useful to them.”

Burwell acknowledged that some critics of the program have questioned whether young people would enroll. “This year, 2.7 million people ages 18 to 34 are signed up for coverage in HealthCare.gov states, and the percentage of new customers in that age range is higher than last year,” she insisted. “The overall percentage of plan selections for those ages remains stable.”