She noted that the new record tally for HealthCare.gov — which serves residents of 39 states — comes despite the fact that the monthly premiums for many individual health plans are rising more sharply for this fourth Obamacare enrollment season than they did in prior years.
"Today's enrollment numbers confirm that some of the doomsday predictions about the marketplace are not bearing out," Burwell said. "Some people asked whether customers would sign up ... and today, we know that answer is 'yes.' "
Last Thursday was the largest day for an open enrollment ever on HealthCare.gov, with 670,000 plan selections.
Thursday had been the original deadline for selecting a plan that begins coverage effective Jan. 1. But federal officials extended the deadline through this past Monday because of the volume of customers.
Open enrollment for 2017 plans, which began Nov. 1, runs through Jan. 31 — 11 days after Trump is scheduled to be sworn in as president.
Burwell on Wednesday said that more than 30,000 people who have called the customer help line at HealthCare.gov have asked whether they should sign up in a plan given the election of Trump, who has vowed to repeal Obamacare.
"Again, our answer is 'yes,'" Burwell said. "The coverage people sign up for today is a contract for 2017."
Trump has said he wants to replace Obamacare after its repeal with something "better," but it's not clear what form such a replacement would take. Congressional Republicans have discussed the idea of repealing several key parts of Obamacare, but delaying their repeal for two years or more to give Congress time to craft a replacement.
Experts agree that a wholesale repeal of the law without replacement would lead to losses of coverage for 20 million people or more. And a number of groups have warned that even if a repeal is delayed for several years, there could be losses of coverage for millions of people in the individual marketplace because insurers would be apt to exit that market because of uncertainty over what the replacement would look like.
Not included in Wednesday's tally were the sign-up numbers from Obamacare exchanges run by 11 individual states and the District of Columbia.
Also not included are the millions of automatic re-enrollments expected to be performed for existing HealthCare.gov customers who did not actively shop for a plan yet, and who did not affirmatively opt out of coverage for 2017.
"There will be millions more [customers] than the number we're talking about today," Burwell said. She noted that HHS has expanded its outreach efforts through a variety of social media platforms, including Instagram and Facebook, in an effort to get remaining uninsured people signed up into coverage.
During the prior open enrollment season, for 2016 coverage, all of the government-run Obamacare exchanges signed up a total of 12.7 million customers.
Burwell earlier this year estimated that 13.8 million people would sign up on those marketplaces for 2017 coverage by the end of January.
"We're sticking with our projections," Burwell said Wednesday when asked how the latest HealthCare.gov tally had affected her outlook.
Andy Slavitt, the acting administrator of the federal Centers for Medicare and Medicaid Services, said he believed the increase in sign-ups was due to growing awareness among the remaining uninsured about the affordability, for many of them, of the Obamacare plans.
"When the existing uninsured population learns about the affordability of coverage, and the choice of coverage, that really does drive their decision," Slavitt said.
Although many Obamacare plans will see their monthly premium prices increase by double-digit percentage amounts in 2017, a large majority of people who are eligible to buy those plans on HealthCare.gov and the state-run exchanges will be able to find a plan that will cost them less than $75 per month, officials have said.
That reflects the fact that most Obamacare customers qualify for federal tax credits to lower their monthly premiums. Those subsidies are available to people with low and moderate incomes.
Under the ACA, most Americans are required to have some form of health coverage of pay a tax penalty, which in 2017 will be as high as 2.5 percent of adjusted household income.