For the first time, 1 in every 3 people with Medicare is enrolled in Medicare Advantage, the private Medicare plans that have played an increasingly large role in the Medicare program over the past decade, according to a new analysis from the Kaiser Family Foundation.

Medicare Advantage enrollment has more than tripled since 2004, reaching 19 million, or 33 percent of enrollees, in 2017, the analysis shows. That milestone is up from 17.6 million beneficiaries (31% ) in 2016, and 11.1 million beneficiaries (24%) in 2010, the year in which Congress reduced payments to Medicare Advantage plans as part of the Affordable Care Act. The Congressional Budget Office has projected that enrollment in Medicare Advantage will continue to rise, reaching 41 percent of all beneficiaries by 2026.

Medicare Advantage enrollment continues to be highly concentrated among a handful of insurers, both nationally and in local markets, the new analysis shows. UnitedHealthcare and Humana together account for 41 percent of enrollment in 2017, for instance, and in 17 states one company has more than half of all Medicare Advantage enrollment – an indicator that these markets may not be very competitive. (Antitrust concerns – in Medicare Advantage and other insurance markets – scuttled recent proposed mergers between Humana and Aetna and Anthem and Cigna.)

The new analysis, Medicare Advantage 2017 Spotlight: Enrollment Market Update, examines trends in this market, including premiums and cost sharing; enrollment and market penetration at the state- and large metropolitan county-level; Medicare Advantage enrollment by firm nationally; and market share of the top three Medicare Advantage firms by state.