Kathleen Sebelius, the former Health and Human Services secretary who oversaw Obamacare’s rollout, said that while the law has brought coverage to about 20 million people, the new insurance exchanges it created are still vulnerable.

“The new markets are fairly fragile,” Sebelius said in an interview on Bloomberg Television. “Insurers are moving in and out.”

The 2010 Patient Protection and Affordable Care Act, known as Obamacare, created markets for consumers to shop for individual health insurance policies and provided subsidies to help many afford coverage. In recent weeks, insurers including UnitedHealth Group Inc. and Humana Inc. have announced plans to retreat from the markets for 2017 because they’re losing money.

“You’ll see some push and pull over the next couple of years,” Sebelius said.

Sebelius, a former Kansas governor, joined the Obama administration in 2009. She resigned in April 2014, after healthcare.gov, the federal website for enrolling in the ACA’s plans, was introduced with errors that prevented many people from initially signing up for coverage. Eventually, after the glitches were fixes, more people signed up than expected in the market’s first year.

In Wednesday’s interview, she repeated her assessment that the law, overall, has been a success, and pointed out that health-care costs are rising more slowly than before the law was passed.

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