"As of this week, 400,000 additional Americans have signed up, and we expect that number to continue to grow," Sebelius said during a Senate Finance Committee hearing on Obama's proposed budget.
The number does not reflect the number of people who have paid their first month's premium, which guarantees that they have coverage. Sebelius has previously cited insurance company estimates that between 80 percent and 90 percent of enrollees are paying their premiums.
Federal officials have said anyone who tried but was unable to sign up by the deadline will have until April 15 to finish their applications. Starting this year, most people must carry health insurance or they will owe a fine.
At the hearing, Sebelius pushed back against characterizations that the extra two weeks represented an "extension" of the open enrollment period. Rather, she suggested it was merely a common-sense step aimed at helping only those consumers who tried but failed to meet the deadline.
"I believe in customer-friendly operations," she said.
Congressional budget analysts had expected 6 million people to sign up for private insurance plans this year under the health-care law, a projection that was scaled back from 7 million after the disastrous rollout of the marketplaces.