WASHINGTON (REUTERS) - About 3.3 million people enrolled in private Obamacare health coverage through new US healthcare marketplaces from Oct. 1 to Feb. 1, the Obama administration said on Wednesday.

The data also shows that the number of young adult enrollees aged 18 to 34 rose slightly to 25 per cent of the total enrollment population.

Participation among young adults could be crucial to the success of President Barack Obama's signature healthcare law because they tend to compensate for higher risks from older or sicker policyholders who represent three-quarters of enrollees.