A quirk in the laws governing Medicare and Social Security will expose millions of Americans to a staggering 50 percent increase in their premiums for the part of Medicare that covers doctors’ bills, known as Medicare Part B. It is imperative that Congress pass legislation to protect low- and middle-income people who cannot pay that much.

Under a 1997 law, premium payments must cover 25 percent of the projected per capita costs for Part B. The premiums, which can rise and fall from year to year, are usually deducted from beneficiaries’ Social Security payments each month. A “hold harmless” provision guarantees that for most people the dollar amount of a premium increase cannot be so big that they are left with a Social Security check that is less than that of the year before. The goal is to ensure that beneficiaries, most of whom have modest incomes, don’t have less money to live on.

The problem is that Social Security recipients will not get a cost-of-living increase in 2016, but Part B premiums are projected to rise. The roughly 70 percent of beneficiaries who are “held harmless” will pay the same premium as last year. That means the increased cost will have to be made up by the other 30 percent, because of the rule that premiums must cover one-quarter of Part B costs. This group includes 2.8 million new enrollees, 1.6 million people who don’t collect Social Security benefits and 3.1 million higher-income beneficiaries.

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All together, more than seven million people will have to pay the premium themselves; millions of others are covered by state Medicaid programs, whose budgets will strain to meet the added costs.