At the main public library in downtown Washington, Anthony Cook, 33, who runs a startup dog-walking business, signed up for a health plan Friday through the D.C. exchange, less than 72 hours before the Feb. 15 open-enrollment deadline. He had tried twice before but was unsuccessful because of technical problems with the exchange website.

He signed up, he said, because he didn't want to incur the Affordable Care Act tax penalty for being uninsured. He chose a bronze-tier plan with a $5,000 deductible, though he wasn't happy about that high deductible. “This plan had one of the lowest monthly premiums,” he said. “I just started a business, and I have to keep costs low.”

The second open-enrollment period under the Affordable Care Act closed Sunday with more than 10 million enrollments. Reform supporters cheered the gains but cautioned that hurdles remain, most notably a U.S. Supreme Court case that could strike down premium subsidies in up to 37 states.

“Growth has been steady, but certainly I think we're seeing the effect of a different population this year looking to become newly insured,” said Elizabeth Carpenter, a director at the Washington-based consulting firm Avalere Health. “Last year, you had a lot of folks who had been unable to access coverage for a long period of time who were very enthusiastic about signing up.”

Enrollment topped the cautious 9.1 million projection announced by HHS. It looks unlikely that consumer enrollment will reach 12 million, the revised estimate issued by the Congressional Budget Office, though the final enrollment total will grow because customers in the process of signing up at the deadline will be given time to complete their applications.

Wild cards

Two wild cards are how the U.S. Supreme Court will rule on premium subsidies and whether auto-renewed customers will pay their premiums.

Still, it will be a tall order to reach the CBO's 2016 estimate of 21 million enrollees and its 2017 estimate of 25 million, which were based on next year's stiffer tax penalties for not buying coverage.

“It's ramping up more slowly than the Congressional Budget Office projected, but I'm not necessarily surprised by that,” said Larry Levitt, senior vice president for special initiatives at the Kaiser Family Foundation. “It's a big, complicated program.”

HHS announced last week that 7.8 million people in 37 states had enrolled in coverage for 2015 as of Feb. 6, using the HealthCare.gov website.

Enrollments spiked by more than 50% in the first week of February, suggesting there would be a surge of new enrollments in the final days of the second period. State-run exchanges had enrolled more than 2.4 million people heading into the final week. But those figures don't take into account whether applicants have paid their first month's premium.

“We've seen a surge, but it's definitely not as busy or as crazy as the final few days last year,” said Sonya Vasquez, health coverage policy director for Community Health Councils in Los Angeles.

One wild card is automatic renewals. Most 2014 exchange customers who did not shop for coverage were automatically re-enrolled. It remains to be seen how many of those customers will continue paying their monthly premiums, said Katherine Hempstead, who oversees coverage programs for the Robert Wood Johnson Foundation. “My suspicion is a pretty high percentage of them will stay in a plan, but they won't all,” she said.

Polling by Gallup found that 12.9% of U.S. adults reported being uninsured at the end of last year, down from 18% prior to the launch of the exchanges. Studies have estimated that roughly 10 million uninsured Americans have gained coverage either through Medicaid expansion or exchange plans.

But the high-deductible plans prevalent on the exchanges may offer coverage that's essentially useless for many lower-income people, cautioned Jeff Goldsmith, a veteran healthcare consultant based in Charlottesville, Virgina. “What percentage of the people with a $5,000 deductible have the $5,000?” he asked. “We don't know.”

Nevertheless, many people who have signed up for private coverage or Medicaid have expressed relief, enrollment workers say. Michael Charles, 58, who recently became unemployed and uninsured, on Friday signed up for Medicaid at the downtown Washington library. He said getting a Medicaid card may have saved his life because he suffers from severe depression.

“I was just lost, feeling down and confused and I didn't know what to do,” he said. “Then I heard on the radio about the (ACA) deadline.” He checked and found out that the mental health therapist he's been seeing for the past 12 years accepts Medicaid. “I had tears in my eyes when they said my application went through.”

Unpredictable politics

Coverage expansion in the states has been uneven. Many observers expected states running their own exchanges to see higher enrollment rates because their elected Democratic leaders promoted sign-ups. But that hasn't necessary panned out. Less than a quarter of the eligible population has obtained coverage through the exchanges in Colorado, Maryland and Nevada, according to the Kaiser Family Foundation.

In contrast, Florida, where Republican elected leaders are hostile to the law, has seen roughly 1.4 million enrollments in private plans for 2015 coverage. That's more than half of the eligible population. Similarly, North Carolina has topped 500,000 enrollments, nearly half of its potential customer base.

Novant Health, a not-for-profit hospital system based in Winston-Salem, North Carolina, held seven enrollment events during this open-enrollment period, and sent out 80,000 letters and 220,000 e-mails to consumers alerting them to coverage options. Bob Seehausen, Novant's senior vice president for business and development, said people were better informed and the politics were less toxic than during the first year of exchange operations.

“The naysayers, that sort of drumbeat, is dying down a bit as people are getting coverage,” he said.

But there are storm clouds ahead. A Supreme Court ruling in King v. Burwell to strike down premium subsidies in states that have not set up their own exchanges could result in 8.2 million individuals losing access to $28.8 billion in subsidies in 2016, according to the Urban Institute. “If the court sides with the plaintiffs in King, I think the individual markets will melt down fairly quickly in the federal marketplace states,” Levitt said.

There's widespread speculation that HHS might announce a special enrollment period to dovetail with the tax filing deadline of April 15. That would allow people who are just becoming aware of the ACA tax penalty to buy coverage before the stiffer 2015 penalty takes effect. HHS did not respond to a request for comment on that possibility.

Jeff Sopka, an insurance broker in Steger, Ill., said one young woman he counseled last year decided to pay the penalty instead of buying a plan that would have cost her $20 a month.

This year she returned and conceded she should have bought coverage. She had suffered a serious accident and incurred high medical bills.

Sopka, a self-described Republican, said enrolling people in exchange plans has been gratifying because he knows they are at least getting preventive health services. “I've changed my thinking completely,” he said. “This is a great thing for people.”

Paul Demko and Virgil Dickson write for Modern Healthcare, a sister publication of Business Insurance.