Centene alone has been responsible for offering coverage in more than 40 previously bare counties in 2018, including 25 counties in Missouri alone.
"This is very good news for people who are getting insurance in this market," Cox said of Centene's move in Nevada, which guarantees that more than 8,000 current Obamacare exchange customers will have an insurer to offer plans.
"It appears at least at this point that most, if not all people will be able to get insurance in this market," she said, noting that she has heard rumors that Wisconsin's Menominee County will soon get an Obamacare insurer.
There were just 47 people who bought an Obamacare plan on HealthCare.gov in Menominee County this year.
"I think what this has shown is that the ACA [Affordable Care Act] is stubbornly failing to fail," Cox said. "The market is stabilizing, and insurers are on track to be profitable this year."
Before Tuesday, there were 14 rural counties in Nevada that did not have an insurer planning to sell Obamacare coverage next year. But Centene said it would sell coverage in those counties, as well as the state's three other counties where it had already announced that it will offer individual health plans.
Garrett Leaf, president of Centene's SilverSummit Healthplan subsidiary in Nevada, said the company "is pleased to serve its current Medicaid members in Nevada, and it looks forward to expanding its product offerings to the Health Insurance Marketplace in 2018."
Heather Korbulic, executive director for Nevada's state-run Obamacare marketplace, said, "We are grateful that SilverSummit has stepped up to the plate, offering relief to thousands of residents who thought they would be deprived of access to health insurance."
Cox said there is an incentive for insurers like Centene "to move into counties where they will be a monopoly."
"They wouldn't be doing this if there wasn't a profit" to be made," Cox said.
The Affordable Care Act requires most Americans to have some kind of health coverage or pay a tax penalty.
The law authorized the creation of HealthCare.gov and state-run insurance marketplaces to sell private individual health plans to people who did not have coverage from other sources.