According to the Department of Health and Human Services, half the uninsured who are eligible for subsidized coverage through the exchanges have refused to purchase it. As a result, those remaining in the insurance pool have tended to be sicker and older — and they’ve used more health services than insurers expected.

How much more? According to the consulting firm McKinsey & Co., insurers collectively swallowed $2.5 billion in unexpected medical expenses from exchange enrollees in 2014.