Some 6.4 million Americans have signed up for Obamacare coverage for 2015, though millions of them were automatically renewed into their existing policies.

The total includes 1.9 million new customers who selected insurance plans through healthcare.gov, as of Dec. 19, federal officials said Tuesday. Another 4.5 million were existing enrollees in the federal exchange, which handles enrollment for 37 states.

But more than 60% of those renewals were done automatically because the enrollees had not revisited healthcare.gov to select a new plan or continue their existing one. The Obama administration gave current enrollees until Dec. 15 to take action before they'd be auto re-enrolled.

Related: Obamacare premiums: Going up unless you shop

Those who were auto-enrolled may suffer from sticker shock when they see their 2015 premiums. Many current plans are going up in price.

That's why the administration strongly encouraged enrollees to shop for 2015 coverage. There are more plans -- and possibly cheaper ones -- to choose from since more insurers have joined the exchanges.

These figures do not include those signing up for coverage on the 14 exchanges being run by states and the District of Columbia. So far, more than 832,000 people have selected plans on the state-based exchanges, according to Charles Gaba, who has been tracking these figures.

Americans surged to sign up on the federal exchange in the days before Dec. 15 so their coverage could begin Jan. 1. Some states, meanwhile, have extended their deadlines. Those signing up must pay their first month's premium for coverage to take effect.

Open enrollment continues until Feb. 15.

The administration has set a goal of 9-plus million enrollees for 2015. Some 6.7 million people were covered through the exchanges, both federal and state, this year.