A Commonwealth Fund report says:

“Using a standard measure of market competition, our analysis finds that 97 percent of markets in U.S. counties are highly concentrated and therefore lacking in significant MA  {Medicare Advantage} plan competition. Competition is considerably lower in rural counties than in urban ones. Even among the 100 counties with the greatest numbers of Medicare beneficiaries, 81 percent do not have competitive MA markets. Market power is concentrated among three nationwide insurance organizations in nearly two-thirds of those 100 counties.”