More than 3 million people are enrolled in a Medicare Advantage plan that is owned by a hospital system or physician group, which is up more than 20% from 2015, according to an Axios analysis of federal data.

What it means: Seniors are increasingly choosing Medicare Advantage instead of traditional Medicare, and hospitals want a piece of those Medicare Advantage dollars as their other revenues stagnate or grow slowly.

Context: Seniors often switch to Medicare Advantage plans for perks like gym membership or vision coverage, in exchange for agreeing to a more limited network of hospitals and doctors.

The details: We tracked Medicare Advantage enrollment growth for 43 hospital systems and physician groups from 2015 to 2018 using data from the Centers for Medicare & Medicaid Services. Here's what we found:

• More than 3 million people have a Medicare Advantage plan owned by a hospital or doctor system as of January 2018, compared with 2.5 million in 2015.
• Kaiser Permanente, the dominant provider and insurance system on the West Coast and in the Mid-Atlantic, covers more than half (almost 1.6 million) of that total.
• The 10 systems with the most Medicare Advantage members represent a vast majority (84%) of the enrollment in provider-owned plans.

Yes, but: The 3 million people in provider plans only represent about 1 out of 7 Medicare Advantage enrollees. And the 20% growth rate for those plans was actually slower than the 24% enrollment uptick for the overall Medicare Advantage program over the same period.

• This means most people are still buying Medicare Advantage plans from the big for-profit players like UnitedHealth Group, Humana and Aetna.
• But health care is local. The prevalence of provider-owned Medicare plans indicates hospitals and doctors with strong brands continue to attract people who otherwise would go to a large insurance company — leading to new revenue and profit.

Go deeper: The shift in Medicare spending.