(Andrew Harrer/Bloomberg)

A total of 2.1 million Americans signed up for plans on the federal and state health insurance exchanges, Health and Human Services Secretary Kathleen Sebelius told reporters in a conference call Tuesday, roughly half of those enrolling through the federal marketplace.

"The new law is transformational for our entire health-care system, and for millions of Americans who finally have health-care security," Sebelius said, adding that 3 million young adults have been able to stay on their parents' health insurance under the law. "Tomorrow is New Year’s Day, and it is a new day in health care for millions of Americans."