Medicare Advantage has surpassed original Medicare in enrollment, a significant milestone for the private-plan alternative to government-run health insurance for people 65 and over.

As of January, 30.19 million of the 59.82 million people with both Medicare Part A and Part B, or 50.5%, were enrolled in a private plan, according to an analysis by KFF, a nonprofit organization focused on health policy, of newly released data from the Centers for Medicare and Medicaid Services. That’s up from just 19% in 2007. Medicare beneficiaries include older adults 65 and over and people under 65 with certain disabilities.

Also known as Part C, Medicare Advantage is administered by private carriers approved by the federal government to provide Part A and Part B coverage. The plans have grown in popularity in large part because they provide coverage at a lower cost to many consumers than original Medicare, which is run directly by the federal government.

Many Medicare Advantage plans have $0 premiums, and in addition to Part A hospital coverage and Part B outpatient coverage, most include Part D drug coverage and certain supplemental benefits, like gym memberships, at no additional charge. (Regardless of their coverage, all Medicare enrollees pay the Part B premium, which is $164.90 a month for 2023, or more for high-income beneficiaries.) Medicare Advantage plans are often aggressively marketed to beneficiaries.

However, Medicare Advantage plans involve trade-offs. Many are structured as HMOs, where patients need to stay within the plan’s network or pay more for care outside. On original Medicare, by contrast, you can visit any doctor in the country who accepts Medicare. While Medicare Advantage plans can be a cost-effective option for the healthy, they can involve high out-of-pocket costs if you get sick or seek care outside the network. What’s more, many require prior authorization for certain services and referrals to specialists.

Beneficiaries can switch their coverage each fall during Medicare’s open enrollment period, which runs from Oct. 15 to Dec. 7. While you can always switch from Medicare Advantage to original Medicare, it isn’t always easy to buy a stand-alone Medigap supplement plan to cover what original Medicare doesn’t. Medigap policies are generally medically underwritten, which means that outside of certain circumstances and a handful of states, you can be denied coverage on the basis of serious health conditions like cancer. The inability to buy a Medigap policy could in effect lock some beneficiaries into keeping their Medicare Advantage coverage even if they’re unhappy with it.