"These consolidations are expected to shift a large number of beneficiaries into lower-cost plans, driving down average premiums in the market by 2 percent from $39.88 in 2014 to $38.95 in 2015. However, significant premium variation exists among the top PDPs by enrollment," Avalere said, noting that some plan premiums will fall by as much as 30 percent while others will increase by 50 percent as the top competitors attempt to fine-tune their customer base.

"Despite sizable premium increases from some sponsors, there continue to be many low-cost options available for consumers — in 2015, five of the top ten PDPs will have average premiums below $30."

Beneficiaries will be shouldering more of the cost burden, Avalere predicted, in the wake of premium declines. The number of zero-deductible plans next year will decrease to 42 percent from 47 percent this year, Avalere said, and Part D sponsors will "continue to move away from offering coverage in the gap due to the continued closing of the donut hole mandated by the Affordable Care Act. As in previous years, about three-quarters of PDPs will not offer coverage of drugs in the Part D coverage gap next year."

Sponsors are combining enhanced MA plan offerings "to reduce the number of PDPs in their books of businesses that offer coverage of drugs in the Part D donut hole. Still others may be responding to CMS' indications that it may soon impose stricter limits on the number and type of PDPs sponsors can offer in the future under the so-called meaningful differences policy," Avalere said.

"When seniors choose a prescription drug plan for 2015, they should carefully look at their options to select the plan that best meets their health and financial needs," said Christine Harhaj, senior manager at Avalere Health. "While beneficiaries will welcome lower premiums, they will need to look at other facets of benefit design including how their medications are covered, cost-sharing responsibilities, and total out-of-pocket spending."