The state’s largest provider of subsidized health insurance is freezing enrollment because its claims this year were costlier than expected, largely because of price spikes in specialty drugs, according to its chief executive officer.

Lewiston-based Community Health Options announced Wednesday that it will stop accepting new members for individual coverage after Dec. 26 – more than a month before open enrollment ends for the Affordable Care Act’s health insurance marketplace. Group enrollment will not be affected.

CEO Kevin Lewis said the co-op’s reserves can cover all current and new members but it had to respond to the higher-than-anticipated claims and its rapid growth.

“We’re putting our finger on the pause button,” he said. “We have to forecast responsibly and insure for the worst case scenario.”

Community Health Options has experienced dramatic growth since its inception two years ago, unlike many of the 23 insurance co-ops created to implement the Affordable Care Act. About half of them will have stopped offering insurance by the end of the year.

In its first year, Community Health Options insured 37,000 Mainers and was the only profitable insurance co-op in the country. This year, it expanded into New Hampshire while growing to 60,000 members in Maine, taking up 80 percent of the marketplace for individual plans, the most dominant market position of any state.

Lewis said the co-op has acquired 5,000 new members since the start of open enrollment on Nov. 1 and expects to have 1,000 more by Dec. 15, the deadline for coverage starting Jan.1. Open enrollment ends on Jan. 31.

Lewis said the co-op plans to open back up to new members as soon as possible and that could be before open enrollment begins in November 2016 for coverage in 2017.

Experts aren’t taking the situation as a bad sign for the co-op and said it makes sense for a small, fast-growing insurer.

“When you have such rapid growth, there’s some concerns about whether you have enough reserves for claims,” said Deborah Chollet, a senior fellow at Mathematica Policy Research in New Jersey.

“The big question is, do we have enough surplus if something cataclysmic were to happen? Do we have enough cash on hand?” she said.

The news may mean some Mainers have to act more quickly to get the insurance plan they want – or rethink staying with Community Health Options – but the situation could be worse, said Emily Brostek, executive director of Consumers for Affordable Health Care in Augusta.

“Obviously it’s a concern for us that this is something that they’ve done, but I also think it’s pretty clear that they’re proactively working with the (Maine Bureau of Insurance) to make sure they remain healthy and financially viable as a company and that’s a good thing,” she said.

Brostek said all insurance companies struggle with having to set their premiums well in advance and can’t adjust them based on what happens that year.

One recent trend that Lewis pointed to as a major culprit for the higher claims is a jump in the cost of pharmaceuticals, especially specialty drugs.

The issue caught the public’s attention this fall when a young CEO purchased a drug used, among other ways, to treat patients with HIV and AIDS, and jacked up the price from $13.50 to $750 per pill.

Maine Sen. Susan Collins is leading an investigation into why several pharmaceutical companies, including the aforementioned Turing Pharmaceuticals, have suddenly increased the prices of certain drugs.

Hearings before the Senate’s Special Committee on Aging began Wednesday.