Large insurance companies are bowing out of health care exchanges citing financial losses. Wonkblog's Carolyn Johnson explains. (Daron Taylor/The Washington Post)

This story has been updated.

Aetna, the nation's third largest health insurer, announced Monday night the most significant departure yet from the marketplaces set up by President Obama's signature health care law. The company, citing $430 million in losses selling insurance to individuals since January of 2014, will slash its participation from 15 states to four next year.

Aetna's exit comes amid a string of announcements that large insurers are losing money selling their plans in the marketplaces, raising doubts about the stability and future of the business.

Chief executive Mark Bertolini said in a statement that there are not enough healthy people to financially offset those with major health problems who require high-cost care.

"As a strong supporter of public exchanges as a means to meet the needs of the uninsured, we regret having to make this decision," Bertolini said in a statement. “The vast majority of payers have experienced continued financial stress within their individual public exchange business due to these forces."

Aetna's announcement comes on the heels of an announcement by Anthem that, in a reversal of expectations, it is now projecting mid-single digit losses on the individual plans it sells on the exchanges for 2016. Humana said it would dial back its participation on the exchanges from 15 states to 11 earlier this month. UnitedHealth Group plans to remain on "three or fewer exchange markets," its chief executive, Stephen Hemsley, said on an earnings call in July. Cigna has said that it is losing money on the exchanges, but the insurer is planning to expand its marketplace presence to three new states in 2017.

Katherine Hempstead, a senior adviser at the Robert Wood Johnson Foundation, said that these national carriers haven't traditionally been the biggest part of the exchanges. She added that while choices and competition will diminish in certain parts of the country — she pointed to South Carolina and Arizona — she didn't see this as a death knell for the exchanges.

"I think the market could survive without these guys," Hempstead said. "Obviously, it would be better to see lots of people seeing a lot of opportunity in this space. ... But I don't think it's a chapter in a Greek tragedy."

The announcements also come as four of the major insurers are in a battle with the Obama administration. The Justice Department has blocked two proposed mergers between Aetna and Humana and Anthem and Cigna; the companies are fighting the decisions. Anthem has said that if its proposed deal with Cigna is allowed to go through, it will increase its exchange offerings to nine additional states.

Caroline Pearson, a senior vice president at Avalere, a health-care consulting firm, said that the lawsuit may have affected the timing of Aetna's announcement, but not the underlying facts about the viability of the marketplaces.

"Aetna may have been holding tight and keeping their mouth shut about how problematic the market is because they were sort of involved in the merger and waiting to hear" from the Justice Department, Pearson said. "But I think the facts in the exchanges are sufficiently dire that all the major carriers need to be assessing how they can be profitable or whether they can continue to participate."

One major issue is the risk pool — the balance of healthy and sick people who incur major medical costs. An analysis by the Centers for Medicare and Medicaid Services released last week showed that the per-month medical costs of members on the exchanges each month had barely budged between 2014 and 2015.

Because this was much lower than the growth of medical spending for the overall insurance market, it appeared to show that healthier people were signing up. But Pearson argued that the data only suggest the risk pool is not getting worse, not that it is improving. The people with the most medical issues were expected to sign up for insurance first. The idea was that the pool would become more balanced over the years as more healthy people signed up. The CMS analysis was also quickly rebutted by insurers.

“This is an overly optimistic assessment of a market that continues to undergo significant changes for both consumers and health plans," Marilyn Tavenner, president of America's Health Insurance Plans said in a statement. "The reality is that the risk pool has not significantly improved. That is a serious concern.”