A typical senior with a Medicare Advantage plan has access to just 43 percent of physicians in his or her county, a new analysis by the Kaiser Family Foundation has found.

Kaiser on Thursday released what it said was the first study to compare the size of networks available without extra fees to Medicare Advantage subscribers, said lead author Gretchen Jacobson, a health economist and expert in Medicare, the government insurance program for seniors and the disabled.

Medicare Advantage plans are sold by private insurers as an alternative to traditional Medicare, whose subscribers often opt for supplemental insurance plans that cover what Medicare does not.  Advantage plans typically offer lower monthly premiums, but patients have in-network access to fewer doctors and hospitals. They can face high out-of-pocket costs if they see physicians who are not in their networks.

"It's important for people to really understand the trade-off," Jacobson said.

A third of the 58 million people on Medicare this year are enrolled in Advantage plans. Kaiser did a similar study about hospital networks last year that found the typical Medicare Advantage plan's network included about half of all hospitals in its county. Twenty percent of networks did not include an academic medical center.

The new report analyzed data from 391 plans from 55 insurers in 20 counties throughout the United States.  Allegheny County was the only location in Pennsylvania or New Jersey.  It and Mecklenberg County, home of Charlotte, N.C., were the only counties where everyone was in broad plans — defined as networks with 70 percent or more of the county's physicians.  Meanwhile, in Clark County, Nev., where Las Vegas is the county seat,  91 percent of Medicare Advantage subscribers are in narrow networks.  That's defined as including less than 30 percent of physicians. Overall, more than a third of Advantage subscribers had narrow networks.

People in traditional Medicare can go to the vast majority of doctors, the report said.

Total network numbers don't tell the whole story.  Jacobson said her group found that some plans had small numbers of specialists.  On average, the plans included just 23 percent of psychiatrists in a county, the lowest percentage for any specialty. Twenty percent of plans included fewer than five cardiothoracic surgeons and 18 percent had fewer than five neurosurgeons.

"These are specialists that you don't necessarily anticipate needing … but, when you do need them, you usually want a choice," Jacobson said.

In general, cheaper plans had smaller networks.  The monthly premium averaged $4 for narrow-network plans compared with $64 for broad-network plans.

It's not easy for people who are shopping for plans — the annual open enrollment period starts Oct. 15 — to compare network sizes.  Consumers can check a network on a plan's website, but formats are not standardized and there's no way to know what percentage of local doctors are included.  The Medicare Plan Finder lists a total number of providers in the network, but it lumps doctors with hospitals, nurses, skilled nursing facilities, and laboratory testing sites.