Hawaii’s two largest health insurers will increase premiums for seniors with Medicare insurance coverage on Jan. 1.

Premiums for Kaiser Foundation Health Plan Inc.’s dominant Medicare Advantage plans will rise to $143 a month from $129 a month for about 25,000 members on Oahu, Maui and Hawaii island. The benefit packages remain largely unchanged.

Hawaii Medical Service Association is raising rates for its Akamai Advantage plans, which cover 28,100 Medicare members. Monthly premiums for HMSA’s Akamai Advantage Select Plus will rise to $34 from $30; to $65 from $54 for its Akamai Advantage Preferred plan; and to $21 from $7 for its Akamai Advantage Assured option. The company’s two zero-premium plans remain unchanged.

Medicare is a federal health insurance program that primarily covers people 65 years and older, as well as some younger people with disabilities and those with end-stage renal disease.

The premium changes come as Medicare allows beneficiaries to switch coverage and carriers during open enrollment, which runs from Oct. 15 through Dec. 7.

“When you’re living on the margin, even what may look like a relatively small increase, like around $10, still matters,” said Barbara Kim Stanton, AARP state director. “A lot of seniors are on fixed incomes, and their medications are expensive. That’s why they need to make sure that they’re choosing wisely in this open enrollment period and looking for plans to really match what their medical needs are.”

The U.S. Centers for Medicare and Medicaid Services announced Tuesday that Kaiser had withdrawn two Medicare plans for Hawaii island beneficiaries as of Jan. 1. But the HMO said it has informed members that it is simply replacing the plans with similar programs under different names. Kaiser’s 1,525 Hawaii island beneficiaries will have to re-enroll in the new plans.

Kaiser spokeswoman Laura Lott said rising costs caused the increases, which are not related to the recent layoff of 35 employees as part of a company reorganization.

“This is a long-term business strategy. We have to remain competitive in the marketplace, and health care costs do continue to increase,” Lott said. “We hope the change will be seamless and that there will be no interruption in service. This is really to cover costs and keep the lights on.”

Meanwhile, Kentucky-based health insurer Humana Inc. has withdrawn a $49-a-month Medicare plan on Oahu and Kauai for roughly 1,900 members, the federal Medicare agency said.

Humana is replacing the plan with a $32-a-month reduced-benefit option on Kauai, and has several remaining plan options for members to choose from on Oahu as it consolidates its product line, said sales director Nicholas Peterson.

Seniors losing their Medicare Advantage plans also have the option of switching to original Medicare, a fee-for-service health plan that allows members to go to any doctor, hospital or other provider that accepts the federal health insurance program, agency said.

Medicare Advantage, operated by private insurers contracted by Medicare, covers the same benefits and services as original Medicare but often includes prescription drug coverage and benefits such as vision and dental care at an extra price.

Medicare Advantage members must generally stay within the plan’s provider network or pay higher out-of-pocket costs.