A new measure of Affordable Care Act enrollment shows just how far behind the government is in hitting its targets—and how far ahead the state-run exchanges are in getting people to sign up for health insurance.
Only 7.8 percent of the eligible population, or 2.2 million people, had chosen a plan by the end of December, according to a report from the Kaiser Family Foundation.
The Congressional Budget Office had projected more than 3.3 million enrollments by then, or 11.6 percent of the eligible population of 28.6 million, as determined by Kaiser.
Kaiser's report was calculated by taking the number of legal residents who are uninsured or purchase nongroup coverage and don't have access to employer-sponsored coverage, while excluding those with Medicaid/CHIP-eligible incomes or those with incomes below the poverty level in states that did not expand Medicaid.
The Centers for Medicare and Medicaid Services, which last week said enrollment via the exchanges had reached about 3 million, would not comment specifically on these numbers.
"We're thrilled with the high demand," said CMS spokesman Aaron Albright. "We're currently ramping up an aggressive sustained outreach campaign to help get as many Americans signed up for health insurance as possible."
(Read more: How to navigate the Obamacare maze)
For the most part, states that run their own marketplaces have had far more success than those using the federal exchange. Vermont leads the pack, with 33.4 percent of its population enrolled.
"We are pleased that we have successfully covered thousands of Vermonters with Vermont Health Connect plans, but our work isn't done until everyone is covered," said Mark Larson, commissioner of the Department of Vermont Health Access.
(Read more: No Medicaid expansion? There's an Obamacare tax hit)
Washington is second, with 28.3 percent of its eligible population enrolled, followed by Connecticut at 16.7 percent. In fact, the top six states on Kaiser's list all run their own exchanges, and 11 of the top 16 spots are held by such states.
"There are likely multiple reasons why state-based marketplaces are having a little bit greater success," said Jennifer Tolbert, who, as director of state health reform for the Kaiser Family Foundation, compiled the numbers.
"Many of the states at the top didn't suffer from the more significant website problems that HealthCare.gov faced in October and November," she said. "In addition to running their own websites, these sites also have invested a considerable amount of money in outreach and enrollment strategies."
That's no coincidence, as the states that run their own websites are the ones that embraced the ACA, whereas many on the federal website have outright opposed it or taken a hands-off approach.
Oregon (5.4 percent), Maryland (4.4 percent), Hawaii (3.8 percent) and Massachusetts (2.1 percent) are the only state-run exchanges not in the top 16, with Massachusetts landing last on the list. However, Massachusetts instituted its own health-care reform in 2006.
"As an existing exchange, we are in a fairly unique situation that goes beyond the numbers in the Kaiser report," said Jason Lefferts, a spokesman for Massachusetts' exchange. He noted that 97 percent of state residents are already insured. "We had nearly 260,000 existing members in December 2013. We have been able to effectively maintain coverage for everyone, while also providing coverage for 33,000 people who were applying for the first time."
(Read more: Obamacare math may not add up for the young)
Eleven of the 16 state-run exchanges have posted enrollments above the overall average, while only seven states on HealthCare.gov have. Maine leads the federal exchange group at 11.2 percent, which is good enough for a seventh-place ranking on the overall list. Next were Michigan (10.4 percent), North Carolina (10 percent), Idaho (9.9 percent) and Montana (8.6 percent).
As for the worst performers after Massachusetts, South Dakota and Mississippi have each enrolled 2.7 percent of their eligible populations, Iowa is at 2.9 percent, and Oklahoma and North Dakota are each at 3.4 percent.
"I think what these data do highlight is that there's still a lot of work to be done to reach out to all of the people who are eligible for coverage and then get them enrolled," said Tolbert.
Source: *As of Jan. 13, 2014. Source: Kaiser Family Foundation
(Note: The table is based on data from "Health Insurance Marketplace: January Enrollment Report," Department of Health and Human Services, released on Jan. 13, as well as more current data reported directly by state-based marketplaces and "State-by-State Estimates of the Number of People Eligible for Premium Tax Credits Under the Affordable Care Act," Kaiser Family Foundation, Nov. 5, 2013.)
—By CNBC's Jodi Gralnick. Follow her on Twitter @jodigralnick.