Millions of U.S. senior citizens face double digit premium increases for major Medicare plans next year as major insurers raise fees, according to a new study by research firm Avalere Health.

Seven of the 10 top-selling Medicare programs are raising fees between 11 percent to 23 percent, according to analysis from Avalere.

Humana Inc. (NYSE: HUM) and Wal-Mart Stores Inc. (NYSE: WMT) plan to increase premiums of its Preferred Rx Plan by 23 percent to $18.50 per month. Coventry Health Care Inc. (NYSE: CVH)'s First Health Part D Premier and Value Plus Plans are set to rise 17 percent and 18 percent, respectively. Cigna Corp. (NYSE: CI)'s Medicare Rx Plan premiums expected to increase by 15 percent.

Avalere said that consumers can avoid the fees by switching to new, lower cost plans. "Seniors need to carefully assess their options going into this open season to ensure that they have a plan that meets their needs," said Bonnie Washington, senior vice president of Avalere Health, in a statement.

UnitedHealth Group Inc. (NYSE:UNH)'s Medicare Rx Save Plus PDP has fees of $15 per month, one of the lowest in most markets. First Health, Conventry and CVS Caremark also have premiums below $30.

The average premium for Medicare plans will remain unchanged at $30 per month in 2013, a number that Obama officials announced in August.

Shares of Humana were down 67 cents to $69.84 in Wednesday morning trading. Wal-Mart gained 12 cents to $74.38. Cigna was up 18 cents to $47.03. Coventry fell 17 cents to $41.63. United was up seven cents to $56.27.

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