In the final period of open enrollment, from March 2 through April 19, nearly 3.8 million people selected a plan, including 1.2 million people in the crucial demographic of ages 18 to 34.
Overall, 2.2 million people in the 18-34 range signed up, relieving fears that too few young people would enroll and overburden the insurance market with older, more expensive patients.
An additional 3 million young adults under the age of 26 have been able to stay on their parents plan, HHS said, and 4.8 million newly qualified people signed up for coverage under Medicaid and CHIP programs, totaling 15.8 million who gained coverage under the Affordable Care Act's provisions.
Outgoing HHS Secretary Kathleen Sebelius attributed the surge to a persistent messaging campaign and the efforts of more than 28,000 in-person assisters and the call centers that supported more than 20 million calls to educate people on their options and help them walk through the enrollment process.
"When individuals saw what the marketplaces offered for them and that they could now afford to purchase coverage for them and their families that made the biggest difference," said Julie Bataille, communications director of the Centers for Medicare and Medicaid Services.
Critics of the law have remained skeptical of the administration's sunny figures. The House Energy and Commerce Committee Wednesday said insurers have provided numbers estimating only 67 percent of enrollees had paid their first month's premiums by April 15.
The White House pushed back on those claims Thursday, pointing out that the numbers used by the GOP were incomplete, but noting that the administration's own figures are still being compiled.
In addition, many of the same enrollees who procrastinated in purchasing their plan until the deadline haven't had their first premium come due yet, and most insurers allow a small grace period for the first payment.