(Andrew Harrer/Bloomberg) (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."

Administration officials said that 3.9 million Americans were determined eligible for coverage through either Medicaid or the Children's Health Insurance Program in October and November, but a group of those were simply renewing.

Roughly 7,000 individuals have called into a hotline set up by the Centers for Medicare and Medicaid Services to be included in a "special enrollment period," according to CMS spokeswoman Julie Bataille, under which they may qualify for coverage retroactive to Jan. 1 because they had trouble enrolling. Bataille said case workers are looking into each caller's particular circumstance, noting, "that doesn’t mean that all of them have been granted."

Another 2,400 individuals have called saying their plans were cancelled, and they have encountered problems getting new coverage, Bataille added.

Officials declined to offered many details about the composition of the group of Americans who met this month's enrollment deadline, including how many are newly insured or the group's demographic breakdown. While it remains unclear how many of the 2.1 million have actually paid their first premium, Bataille said both federal officials and insurance representatives have reminded these individuals they will have to pay at some point in January in order to ensure their plan takes effect.

"We are confident those consumers have selected a plan and know what the next steps are for them in terms of securing coverage," she said.

Bataille added that federal officials are also confident the 2.1 million figure means more Americans, rather than fewer, now have health-care coverage starting Wednesday since it "certainly outpaces anyone who may have lost a plan" this year.