Large numbers of Medicare beneficiaries facing a big shift in coverage for 2019 moved to Medicare Advantage health plans, apparently drawn to their low premiums and steered there by insurers.

But it's left some wondering if more switching could be coming over the next few months, as patients start using coverage that can feature tighter access to doctors and hospitals.

"We don't know what we're going to see in January, February and March when people start wanting to go to the doctor," said Kelli Jo Greiner of the Minnesota Board on Aging. "If they find out their provider doesn't participate … they may want to change plans."

Federal law forced an unusually large number of Medicare beneficiaries in Minnesota to switch coverage going into 2019 as the government eliminated a popular form of coverage called Medicare Cost plans.

Those plans were eliminated in 66 counties in the state where there's significant competition from Medicare Advantage (MA) insurers. It drove a 330,000-person decline in that sort of coverage between December and January, according to data released last week by the federal Centers for Medicare and Medicaid Services. During the same period, MA enrollment grew by more than 200,000 people.

The elimination of Cost plans set up a choice for consumers: Go with an MA plan from a private insurer or revert to the government's original Medicare program.

The shift also set off a marketing frenzy as insurance companies pushed not only their MA plans but also Medicare Supplement policies known as "Medigap" plans and stand-alone "Part D" prescription drug coverage, both of which are popular among people with original Medicare.

Insurance agents say the enrollment numbers released last week show many consumers responded to lower premiums with MA plans.

"A lot of households couldn't afford the supplements — they either couldn't afford them or they wouldn't," said Tom Peterson, an insurance agent with Twin Cities Underwriters in Roseville.

Another factor is that health insurance companies that sell Medicare Cost plans were allowed to steer many beneficiaries to MA plans, said Gretchen Jacobson, a Medicare expert with the California-based Kaiser Family Foundation. She noted that Medicare enrollees still have the chance to make changes.

"One important question is: How much will these numbers change during the next two months when people are able to look at their plan a little more closely and decide if they really want to stick with it?" Jacobson said. Those who lost Cost plans "have a choice and a rare period where they are guaranteed to be issued a Medigap policy if they want it."

MA plans have been gaining market share across the country as the nation's largest health insurers see a growing business opportunity.

Supporters say the trend leads to better coordination of care for patients plus extra benefits with relatively low premiums. Critics fear that too many seniors don't appreciate how the coverage can affect their choice of doctors and prescription drug plans.

For many years, Cost plans have been a big business for the state's three largest nonprofit health insurers — Blue Cross and Blue Shield of Minnesota, HealthPartners and Medica. A Star Tribune review of government data for December and January shows that while the insurers collectively lost about 330,000 enrollees in Cost plans, they grew their MA enrollment by about 135,000 people.

While the numbers suggest a decline in market share for the companies, the loss is mitigated by former Cost plan enrollees who opted for original Medicare and now are buying a Medigap policy from their old insurer.

Blue Cross, which has been by far the largest Medicare Cost plan in the state, lost about 206,000 Cost plan enrollees while adding about 89,000 Medicare Advantage plan enrollees. At the same time, the company says it more than doubled its tally of Medigap customers, which previously stood at about 80,000 people.

When it's all said and done, Blue Cross saw some decline in its Medicare enrollment, but "we're pleased with our retention results," said Monica Engel, president of the insurance company's Medicare business.

Overall Medicare enrollment also declined at HealthPartners and Medica, where officials say the insurers saw a bigger drop in Medicare Cost enrollment than the simultaneous jumps in Medigap and Medicare Advantage. At Medica, increased competition was a factor in the decline, along with the decision to not sell Medicare Advantage plans across all counties, said Tom Lindquist, a senior vice president with the insurer.

While the overall figures show a statewide decrease of 330,000 people in Cost plans, the data release doesn't show precisely where those enrollees went in terms of their coverage, Lindquist noted. So, while the simultaneous jump of more than 200,000 people in MA enrollment might suggest two-thirds of the Cost plan enrollees opted for MA, Lindquist thinks the share is likely between 50 percent and 60 percent. Some of the MA growth, he said, likely came from people who newly qualified for Medicare.

"This is just the first year of the transition, which, as far as I'm concerned, is going to be a two- to three-year process," Lindquist said. "You'll see shifts back and forth between Advantage and [Medigap] as people settle in and figure out what they have."

The enrollment numbers show that Humana and UCare, which have sold Medicare Advantage plans for many years in Minnesota, collectively added more than 62,000 enrollees, with the majority going to Humana. The Kentucky-based health insurer did not comment.

Ghita Worcester, a senior vice president at UCare, said the Minneapolis-based HMO has seen a significant increase in sign-ups since the cutoff for collecting the data released last week. The latest numbers aren't final, Worcester added, because of ongoing enrollment chances.

There's a special "guaranteed issue" right to purchase a Medicare Supplement policy until March 4 for those who lost Cost plans; they also have options related to Part D plans through the end of February. Finally, there's an extra open enrollment period for MA plans that extends to March 31.

The big shift brought new players to the state's Medicare market for 2019. New entrant Allina Health Aetna added 5,200 MA enrollees, the data show, while newcomer United­Healthcare picked up more than 2,300.

MaryJo Webster contributed to this report.

Christopher Snowbeck • 612-673-4744 Twitter: @chrissnowbeck