Affordable Care Act enrollment has accelerated in most states over the past few weeks, but the numbers are still way behind targets.
As of March 1, 14.8 percent of the eligible population—or 4.24 million people—had chosen a plan, according to data from the Kaiser Family Foundation.
The Congressional Budget Office had projected more than 5.7 million enrollments by then, or 19.9 percent of the eligible population of 28.6 million, as determined by Kaiser.
"Projections are constantly changing based on experience," CMS spokesman Aaron Albright told CNBC. "We are focused on enrolling as many people as possible in each state before open enrollment ends on March 31."
Enrollment has been accelerating, as the once problem-plagued Healthcare.gov website and some state exchanges have become more stable and user friendly. And, with 18 days to go in the open enrollment period, that trend may continue, said Jennifer Tolbert, director of state health reform for the foundation.
"I expect there will be a surge in enrollment. If history holds true, we've seen this in other situations. There are a lot of people who have put off signing up and will wait until the last minute," she said.
The foundation calculated 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.
Vermont continues to lead the country by a wide margin, with 54.1 percent of its eligible population having picked a plan. Vermont is one of the few states that has released data on paid enrollments, showing 59 percent of those enrollees had paid their first premium by March 1. It's unknown how many of the unpaid enrollees have been billed.
Washington is second, with nearly 37 percent of its eligible population enrolled, followed by Rhode Island at 27 percent. The top five states on the list all run their own exchanges, but states on the federal exchange are gaining ground. Idaho (21.7 percent), Maine (20.8 percent), Michigan (19.9 percent) and North Carolina (18.7 percent) are all in the top 10.
And of the 17 states that posted enrollment above or equal to the overall average, about half are on the federal exchange.
Massachusetts, which runs its own exchange, is last on the list at 5 percent. However, the state instituted its own health-care reform in 2006, and 97 percent its residents were already insured before ACA.
Rounding out the bottom five are South Dakota (5.7 percent), Iowa (5.9 percent), North Dakota (6.8 percent) and Oklahoma (7.4 percent). All four of those states more than doubled their numbers in January and February compared with the first three months of open enrollment.
In fact, 28 states more than doubled their enrollments in the first two months of 2014, compared with the October through December period. Among the notable increases: Florida jumped from 6.2 percent enrollment to 17.4 percent; Idaho moved from 9.9 percent to 21.7 percent; Mississippi more than tripled enrollments from 2.7 percent to 8.6 percent; and Texas increased from 3.8 percent to 9.4 percent.
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Still, most states are well below their CBO targets, with only Connecticut and Rhode Island having actually reached those projections for the end of February.
Access Health CT CEO Kevin Counihan cited great marketing, consistent website function since Day One and an easy enrollment experience as reasons for Connecticut's success.
Counihan said the exchange scaled back its technical requirements by 30 percent nine months before the launch. "We moved from making it a Cadillac to a Ford Focus," he said. "It's not a very sexy car, but it runs."
(Read more: Obamacare: ACA enrollment just 4.2 million by Feb.)
In fact, things are going so well, he's already looking ahead to the fall enrollment period. "Not to say we're taking our eye off the ball in March. We're not. But this thing is cranking. We need to start focusing on renewing business, making the experience even easier. One of the goals is that it keeps getting easier every year," he said.
(Note: The table is based on data from "Health Insurance Marketplace: March Enrollment Report," Department of Health and Human Services, released on March 11 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.)
UPDATE: The text of this story was updated to include a comment from CMS.
—By CNBC's Jodi Gralnick. Follow her on Twitter @jodigralnick.