The U.S. Department of Health and Health Services (HHS) has bragged about stability in Medicare Part D prescription drug plan prices — but the number of drugs the average plan covers is falling.

Kev Coleman, an analyst at HealthPocket, has quantified the drop in an analysis of data from the Centers for Medicare & Medicaid Services (CMS), the arm of HHS that runs the Medicare Part D plan program and the Medicare Advantage health plan program.

The 2015 private Medicare plan open enrollment period starts Wednesday. 

The Medicare Advantage program the Medicare Part D program give private insurers a chance to sell coverage to people who qualify for Medicare. The performance of the programs is of interest to players in the commercial health insurance market because Congress based many of the commercial major medical insurance provisions in the Patient Protection and Affordable Care Act (PPACA) on Medicare plan market provisions.

Allsup, a provider of Social Security disability and Medicare services based in Illinois, has reported that the base Part D plan monthly premium will increase 2.2 percent, to $33.13 in 2015, from $32.42 this year.

Analysts at a HealthPocket competitor, Avalere Health, noted in September that the total number of Part D plans available will fall 14 percent, to 1,001.

Coleman found that the average Medicare Part D plan will cover 1,418 drugs in 2015, or 3 percent less than the average plan is covering this year. The percentage of covered drugs affected by some kind of usage restriction will increase to 37 percent, from 34 percent. Patients will have to get prior authorization from the insurer to use 22 percent of the drugs, up from 19 percent.

Many Medicare Advantage plans also offer prescription benefits. They will cover an average of 1,485 drugs each, or about as many as they cover this year. The percentage affected by restrictions will increase to 34 percent, from 32 percent.

See also: Consumers failing to identify lowest-cost options in plan choice