What do recent mergers of major health carriers mean for seniors? It might reduce the already-sparse competition that exists among private Medicare health plans, some consumer advocates worry. 

An analysis by the Commonwealth Fund finds that 97 percent of U.S. markets for Medicare Advantage are "highly concentrated," meaning there is not significant competition.

The problem is particularly pronounced in rural areas, but competition is far from robust in densely-populated areas of the country. In the 100 counties with the highest number of Medicare beneficiaries, 81 percent are classified as uncompetitive by Commonwealth. Medicare Advantage, which seeks to provide private alternatives to traditional Medicare, has seen increasing participation in recent years, with 30 percent of seniors opting for the private plans.