While nearly 25 million moved from one source of insurance coverage to another in that time frame, the study "suggests the ACA isn't just shuffling people around in different types of insurance, it is bringing people in who were previously uninsured," said Carman.
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But she also noted that "the vast majority in the reduction in the uninsured was as of May 2014," with a marked drop-off in the pace of reducing the uninsured rate since the first open enrollment period.
Carman said she thinks there could be stronger progress in reducing the number of uninsured in future years. But, she added, "I'm not ready" to make predictions on when that could happen.
The gains seen in 2014—the first year Obamacare exchange plans went into effect—came from a surge of people who most needed insurance and those who were most likely to be influenced by encouragement to sign up for those plans, Carman said.
That year, the health-reform law barred insurers from discriminating against customers with pre-existing conditions and mandated that nearly all Americans have health coverage or be subject to a tax penalty.
"Since then, there hasn't been a thing to really change things," she said.