WASHINGTON _ The Obama administration went to court Thursday to block two major health insurance mergers, siding with consumer advocates and medical groups worried that the consolidation of large national health plans could lead to higher premiums.
The move by the Justice Department, which has been expected for weeks, will prevent Anthem Inc.'s purchase of Cigna Corp., a combination that would have created the nation's largest health insurer.
And it will stop Aetna Inc.'s bid to acquire Humana Inc., a merger that would have combined the nation's third and fifth biggest health plans.
The lawsuits are unlikely to end maneuvering in the health insurance industry, as health plans try to bolster their positions in a fast-changing industry still being reordered by the 2010 Affordable Care Act.
Three of the four companies said they plan to challenge the Justice Department's move.
In a joint statement Thursday, Aetna and Humana said they would "vigorously defend" their deal.
"A combined company is in the best interest of consumers, particularly seniors seeking affordable, high-quality Medicare Advantage plans," the companies said.
Anthem said Thursday it was "fully committed" to challenging the Justice Department suit in court "but will remain receptive to any efforts to reach a settlement with the DOJ that will allow us to complete the transaction."
The company called the suit "an unfortunate and misguided step backwards for access to affordable healthcare for America."
Cigna said it was "evaluating its options" and does not believe the deal could close until 2017 at the earliest, "if at all."
It is unclear how long the legal wrangling could take and whether the showdown between the administration and four of the nation's five largest insurers could have broader impacts on the nation's healthcare system.
For the past several years, the administration has leaned heavily on insurers to help implement the 2010 health law, often called Obamacare.
Also unclear is whether the administration's aggressive stance on the proposed insurer mergers will extend to consolidation in other health sectors.
For years, hospitals and physician practices have been merging around the country, creating increasingly concentrated markets dominated by one or a handful of huge medical systems.
Many experts believe this consolidation is at least as harmful to consumers as insurance mergers.
"This is a huge issue for our country," said Martin Gaynor, a Carnegie Mellon health economist who has researched consolidation.
"If we don't do something about this now, we will have an even more consolidated, expensive, unresponsive health system than we have already, and once this happens, it will be extremely difficult, to impossible, to change."
There is growing evidence that this so-called provider consolidation is driving up prices for medical care, perhaps even more than consolidation in the insurance industry.
Americans already pay the highest prices for their care in the world, surveys show.
But blocking hospital mergers or preventing medical systems from buying up physicians' practices is much more difficult than taking on a national merger of two insurance companies, said Dr. Robert Berenson of the Urban Institute, a Washington, D.C.-based think tank.
It is far more difficult once hospitals and physician practices have joined together.
"You can't unscramble the egg," Berenson said.
Anthem stock was up about 1.5 percent in early trading Thursday ahead of the announcement. Cigna and Aetna were up less than 1 percent, while Humana stock was down slightly.
