More than 12 million Americans chose health plans for this year through the Affordable Care Act’s insurance marketplaces, according to new federal data that show an increase in the number of returning customers and a significant drop in new participants.

The numbers released Wednesday morning, a final report on the most recent enrollment season for ACA coverage, echo preliminary findings last month that the number of people signing up for health plans declined for the first time since the marketplaces opened three years ago. A total 12.2 million enrolled compared with 12.7 million a year ago.

The decrease occurred primarily in the 39 states relying on, the federal insurance exchange, where the number of people signing up fell by 400,000. In the remaining states, which run their own marketplaces, 3 million people signed up, a decrease of 100,000 from 2016.

The unprecedented drop coincided with the beginning of the presidency of Donald Trump, an avowed foe of the 2010 health-care law enacted by a Democratic Congress. Progress reports earlier in the three-month enrollment season that ended Jan. 31 had been showing higher participation than in the past. But the pace of sign-ups slowed significantly toward the end, when the Trump administration cut advertising encouraging Americans to get coverage through the marketplaces and flip-flopped on other outreach activities.

Hours after his inauguration, the president also signed as one of his first actions: a broad executive order directing federal agencies to ease the burden ACA regulations place on consumers and the health-care industry.

The new data show that most of the people who signed up for ACA coverage — intended for Americans without access to affordable health benefits through a job — continue to get considerable federal help to pay for private health plans. Some 84 percent, or about 10 million people, qualify for tax credits the law provides; that is just slightly less than a year ago. The tax credits for those who qualify average $383 per month and typically cover nearly three-quarters of the price of their insurance premiums.

The figures reveal that the ACA did not make headway this year in its goal of increasing the share of young adults, whom insurers are eager to attract because they tend to be healthy and inexpensive to cover and offset the expenses of older and sicker people in a health plan's insurance pool. The percentage of people younger than 35 this year is 36 percent, unchanged from last year.

The enrollment figures released Wednesday reflect the number who signed up for coverage. People become insured once they begin to pay their monthly premiums.

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