"We're off to a strong start," said Burwell during a conference call with groups that promote Obamacare enrollment.
Open enrollment for 2016 plans closes Jan. 31. Under the Affordable Care Act, nearly all Americans must have some form of health coverage or risk a fine, which in 2016 will be the higher of $695 per adult or 2.5 percent of household income.
Burwell was particularly pleased by the strong surge in enrollment among young adults. "We're excited about this," she said. "It's good news. It means a younger risk pool."
The younger an insurer's risk pool, or group of customers, generally speaking, the better chance plans have to turn a profit. Younger enrollees typically spend more in premiums than they use in health benefits, in contrast to older, sicker enrollees, who can use more benefits than what they pay in premiums.
The lag in getting relatively large numbers of young people to sign up in Obamacare plans during the first two enrollment seasons, for 2014 and 2015, has worried advocates of health-care reform and insurers.
Larry Levitt, a senior vice president and Obamacare expert at the Kaiser Family Foundation, said that increasing the number of young adults could assuage those fears.
"The first people in the door were older and sicker, so increasing enrollment is key to balancing out the risk pool," Levitt said. "The increase in sign-ups we're seeing so far this year is bringing more young adults into the insurance pool, and probably more healthy people as well."
But Levitt also said he thought the better "apples-to-apples comparison" for the change in youth enrollment is "new plan selections" by people as opposed to all plan selections, which includes both new enrollees and existing customers. All plan selections grew from 1.1 million last season to 2.1 million. But the number of new plan selections rose to 980,000 so far this season, compared to 670,000 at the same point, which Levitt said represents "strong growth, but obviously not doubling."
Burwell said she was also happy about the strong demand in advance of a mid-December deadline to sign up for coverage that would take effect Jan. 1, saying there were "unprecedented numbers" of callers to HealthCare.gov's telephone help line and to its website.
Anne Filipic, president of the Obamacare promotion group Enroll America, said, "Today's announcement that more than 8.2 million Americans — an increase of 29 percent over this time last year — have chosen a plan or been automatically renewed on the health insurance marketplace makes clear that the success we've seen under the Affordable Care Act is not just holding steady: it is growing and thriving."