A small number of insurers controls a big chunk of the market for Medicare Advantage health plans, according to a new Kaiser Family Foundation report.

The nonprofit foundation said three or fewer companies dominate the market in every state except New York for the plans, which are privately run versions of the government's Medicare program that provides coverage for people over age 65 and the disabled.

Medicare Advantage plans are subsidized by the government and offer basic Medicare coverage topped with extras or premiums lower than standard Medicare rates.

In 14 states and the District of Columbia, one company enrolls more than half of all Medicare Advantage customers, Kaiser said.

This means that as health care reform unfolds, decisions made by a small number of companies will have a large impact on the type of plans and premiums available over the next several years, said Tricia Neuman, a foundation vice president and director of its Medicare Policy Project.

Health care reform aims to provide coverage for millions of uninsured people and will pay for that in part with phased-in Medicare Advantage funding cuts. No cuts were implemented this year, but Medicare will freeze the maximum amount it pays plans next year.

Kaiser, which released its report Tuesday, said two insurers — UnitedHealth Group Inc. and Humana Inc. — control 33 percent of total Medicare Advantage enrollment nationwide. That rose to 11.1 million people this year, up 6 percent from 2009.

While a few companies control most markets, they offer many choices for customers. Kaiser found that the average enrollee has 33 plans available in his or her area.

The average monthly premium paid for a Medicare Advantage plan with prescription drug coverage was $44 this year, up from $36 in 2009.