“Despite concerns that reductions in payments to Medicare Advantage plans enacted in the Affordable Care Act of 2010 (ACA) would lead to reductions in Medicare Advantage enrollment, the number and share of Medicare beneficiaries enrolling in Medicare Advantage plans has continued to climb. Since the enactment of the ACA, Medicare Advantage enrollment has increased by 5.6 million, or by 50 percent. The ACA payment reductions aimed to reduce historical overpayments to Medicare Advantage plans, relative to traditional Medicare. This Data Spotlight reviews national and state-level enrollment trends as of March 2015 and examines variation in enrollment by plan type and firm. It analyzes the most recent data on premiums, out-of-pocket limits, Part D cost sharing, and plans’ quality ratings for Medicare Advantage enrollees. Key findings include:

• Medicare Advantage enrollment has continued to grow and increased in virtually all states in 2015. Almost one in three (31 percent) people on Medicare (16.8 million beneficiaries) were enrolled in a Medicare Advantage plan in March 2015 – up by more than one million beneficiaries from 2014.
• Medicare Advantage enrollment continues to be highly concentrated among large firms, both nationally and in local markets. Together, UnitedHealthcare and Humana account for 39 percent of all Medicare Advantage enrollment, and three other firms and BCBS affiliates account for another 33 percent of enrollment in 2015. In 15 states and the District of Columbia, more than half of all enrollees are in plans offered by one company – an indicator that these markets that may not be very competitive.
• Despite the growth in enrollment in new plan types, most enrollees continue to be in HMOs (64% in 2015). As in prior years, enrollees in HMOs, on average, pay lower premiums and have lower limits on total out of pocket expenses than enrollees in other plan types.
• The share of enrollees in plans with 4 or more stars rose from 37 percent in 2013 to 61 percent in 2015.
• While growth in premiums has been relatively modest, Medicare Advantage plans provide less financial protection to Medicare enrollees than they have in the past. While average monthly premiums for MAPDs (weighted by enrollment) remain relatively flat ($38 per month in 2015, an increase of $3 per month from 2014), they vary across plan types and states…”