Half of Virginia's counties now are on track to have no health insurers offering Obamacare plans in 2018 after an insurer reversed a decision to sell individual health coverage in much of the state.

The pullback by Optima Health in Virginia ends a brief, two-week period in which every county in the United States was projected to have at least one Obamacare insurer next year.

And it reflects ongoing uncertainty among insurers about the stability of the individual health plan market in light of the factors including Trump administration's hostility to Obamacare.

On Wednesday, big insurer Anthem, citing market volatility, said it would slash in half the number of counties in Kentucky where it will sell Obamacare plans in 2018.

Virginia's Bureau of Insurance said 48 counties and 15 cities are now expected to be "bare" of Obamacare plans in 2018.

There are a total of 350,000 Virginians who currently have individual health plan coverage and who live in those bare areas, or have a plan provided by an insurer that will not longer offer coverage next year, according to Sentara Healthcare, the nonprofit health system parent of Optima Health.

Bureau spokesman Ken Schrad said several other ZIP codes also are projected to lack an Obamacare plan option. The bare areas are rural or have low population density compared with the rest of the state.

Asked if the department was confident it would be able to get another insurer to cover the bare counties, Schrad said, "I can't make a prediction as to the success."

"All I can say is the Bureau of Insurance is in serious discussions with carriers to see if the bare counties will have an option on the federal exchange in 2018," Schrad said.

Insurers currently until Sept. 20 to file proposed premium rates for 2018 and until Sept. 27 to sign contracts to sell Obamacare plans.

But Schrad noted that federal regulators had already pushed back the rate filing deadline once this year because of uncertainty in the individual health plan market.

"It's a fluid situation," he said.

Optima Health is one of the insurers talking with the state about picking up customers in some of the bare areas, according to the company.

Michael Dudley, president and CEO of Optima Health, told CNBC on Thursday, "we're very, very sympathetic to" the people who will be potentially left without coverage options.

"It tears us apart," Dudley said.

But he also said that the insurer's decision to scale back its expansion plans were the direct result of Anthem's announcement last month to totally exit Virginia's Obamacare market in 2018. Two other big insurers, Aetna and UnitedHealthcare, had previously announced plans to leave the state's individual plan market.

Dudley said he feared that Optima Health, which now serves only 17,000 or so Obamacare customers, would be swamped by the more than 200,000 such customers now served by Anthem.

While such an influx of customers would bring significantly higher revenue for Optima Health, it also would bring the risk of high health usage by new customers whose costs to the insurer could greatly exceed what they pay in premiums.

"We want to help as many Virginians ... but we have to do it in a way that we think is manageable for us," Dudley said.

Dudley said another "key consideration" for Optima Health was the failure of the Trump administration and Congress to guarantee that the federal government will continue paying insurers billions of dollars in so-called cost-sharing reimbursements. That money reimburses insurers for discounts that they must offer by law to lower-income Obamacare customers for their out-of-pocket health costs.

"The combination of those two things" — uncertainty about CSRs and the potential flood of Anthem customers — "creates a perfect storm," Dudley said.

Optima Health will still sell individual health plans next year in areas where it currently offers Obamacare coverage and where there are Sentara Healthcare hospitals and physicians. The company also expects to significantly add to its current individual plan customers because of the departure of larger insurers.

Optima Health is also asking for steep rate hikes next year, although the company noted that most of its customers receive federal Obamacare subsidies that will greatly limit the effect of the increases.

About 70 percent of Optima Health customers "will have their increases mostly absorbed by the government," the company said.

But for the remaining 30 percent of customers "there will be an average 81.8 percent increase to premiums," Optima Health said.

The company said that about 20 percent of the price hike is how much Optima Health originally planned to raise prices by.

Another 23 percent of the increase reflects uncertainty about whether the Trump administration and Congress will guarantee the CSR payments.

The remaining share of the price hike is the result of "the withdrawal of the national carriers" from the market, the company said.