UnitedHealthcare is trying to grow its already large business selling Medicare Advantage health plans by adding 291 counties to its nationwide service area including seven new counties in Minnesota.

It's the company's "largest footprint expansion in five years," the Minnetonka-based health insurance giant said in a Thursday announcement. The service area will grow by roughly 16%, the insurer said, to a total of 2,117 counties.

UnitedHealthcare's push is just one of several changes for 2021 that insurers are announcing with the start Thursday of the annual marketing season for Medicare Advantage health plans as well as prescription drug plans that provide Part D benefits. Government rules allowed insurers on Thursday to start sharing plan details in advance of the annual open enrollment period, which begins Oct. 15 and runs through Dec. 7.

Broadly speaking, Minnesota seniors won't be confronting steep premium increases, said Kelli Jo Greiner, the Medicare program manager at the Minnesota Board on Aging.

"Premiums for the most part are stable with Medicare Part D and Medicare Advantage plans — we're not seeing a lot of increases," Greiner said.

COVID-19 will make the open enrollment period unusual, as insurers and advisers shift away from in-person sales and education events.

"Safety is our top consideration this sales season, especially to protect Medicare-aged people," officials with UCare, a Minneapolis-based health plan, said via e-mail. "We are offering a plethora of virtual ways to learn about our 2021 Medicare plans."

Medicare Advantage plans are a newer form of Medicare coverage, where beneficiaries elect to receive government-funded benefits via managed care plans run by private health insurers. The market has been a big source of growth and profits for health insurers; the plans often come with lower premiums, but advocates caution that Medicare Advantage plans often provide fewer choices in doctors and hospitals.

People who stay in the traditional Medicare program, rather than opt for Medicare Advantage coverage, often buy supplemental "Medigap" policies to reduce their out-of-pocket expenses. Some in Minnesota still receive their government benefits through Medicare Cost plans, which the federal government has been trying to phase out in recent years.

Minnetonka-based Medica said its service area for Medicare Advantage plans in Minnesota will grow from 29 counties to 47 counties. Medica will continue to sell Medicare Cost plans in 21 counties across the state.

Bloomington-based HealthPartners is expanding its Medicare Advantage service area to six new counties in Minnesota, nine new counties in Iowa and one county in Illinois — an increase across the Midwest region, which includes Iowa, from 57 to 73 counties. The insurer sells Medicare Cost plans in 53 counties across Minnesota, Wisconsin and the Dakotas.

UCare isn't changing its service area, but the insurer is adding new plan options in Minnesota. Consumers will see stable or lower premiums, enhanced benefits and lower copays, the health plan said.

Allina Health Aetna is expanding its Medicare Advantage service area in Minnesota from 12 to 20 counties, said Britta Orr, the insurer's chief Medicare officer. The health plan is a joint venture between the Minneapolis-based Allina Health System and the national carrier Aetna, which is adding 115 counties to its service area across the country.

Overall, Aetna said it will sell health plans in 1,793 counties next year.

Kentucky-based Humana isn't changing its service area in Minnesota, but is adding 39 new counties across the U.S. in 2021 for a total of 2,709.

Humana picked up business when Minnesota's market for Medicare health plans underwent big changes going into 2019 with the elimination of Cost plans in the Twin Cities metro and much of the state. Going into 2020, advocates were concerned about inaccuracies with a Medicare website for comparing plan options.

With the coming open-enrollment period for 2021 coverage, the Medicare website is significantly improved, with better information on Medicare Supplement plans as well as changes in medication formularies, said Greiner of the Minnesota Board on Aging. There are more changes with health plan options for next year, she said, but the overall volume of changes is "modest."

Just over one-third of the 68.6 million people eligible for Medicare opt for benefits through a Medicare Advantage plan and the market share has been growing, says Mark Farrah Associates, a Pennsylvania-based market research firm.

UnitedHealthcare was the largest Medicare Advantage carrier with about 26% of the market as of March, the consulting firm said; last year, United also was the nation's largest Medicare Supplement carrier with about 32% of the market.

In Minnesota, UnitedHealthcare's service area is growing from 43 to 50 counties.

Twitter: @chrissnowbeck