About 3.3 million people have signed up for private insurance through President Obama's health care law as of Feb. 1, according to a new report from the Department of Health and Human Services, but the number is still lagging behind the administration's original forecast.

In September 2013, weeks before the opening of the new insurance exchanges, HHS projected that nearly 4.4 million would sign up for insurance through the exchanges by the end of January.

Also on Wednesday, HHS reported that 27 percent of those signing up for the new exchanges in January were between 18 and 34 years old, up from 24 percent in the prior three months. Cumulatively, 25 percent of sign-ups come from this demographic — which is still well below the 38.5 percent that the administration had previously said were necessary to make the exchanges viable.

It's crucial for insurers to attract a critical mass of young and healthy individuals to offset the cost of covering older and sicker individuals, particularly those with pre-existing conditions.

The data released by HHS did not specify how many of those signing up for insurance have actually paid for their plans, which is typically how insurers measure enrollment. HHS includes all those who have "selected" a plan through one of the exchanges as being enrolled.