Shares of CVS Health fell Friday after its insurance arm Aetna lost ground in the latest ratings of Medicare Advantage plans for seniors.

The stock lost 1.87%, closing at $86.16, while peer Humana, which made big gains in the ratings, gained 11.56% during the session, for a $431.87 closing price.

Insurance plans in the federal program are rated each year on a scale of one to five stars, summing up performance on dozens of quality measures. Seniors can view the star ratings as they shop this fall for next year's health coverage.

Wall Street tracks the ratings closely because plans that achieve four stars or more become eligible for bonus payments worth billions of dollars in total. The 2027 ratings will lead to bonuses in 2028, paid on a per-member basis.

The portion of Aetna membership in a four-star or higher plan shrank compared with last year, per the new 2027 ratings released Thursday after the market closed.

According to Aetna, about 69% of its members are in a 2027 plan with at least four stars, down from about 81% when the 2026 stars were first announced.

The ratings "came in weaker than expected" for CVS, said TD Cowen analyst Charles Rhyee in a research note Thursday evening. Rhyee estimated the "unmitigated impact" from lower year-over-year ratings at $720 million in 2028.

"While disappointing, we see potential for CVS to partially offset through benefit design," Rhyee wrote. "We expect additional color on the Stars outcome, mitigation opportunities, and sizable impacts" when CVS reports earnings for the third quarter.

Aetna President Steve Nelson said the insurer was "encouraged by our performance in areas like clinical quality and operational excellence" in the ratings.

Nelson said Aetna has "confidence in our ability to achieve our goal of returning to appropriate margins while at the same time driving continuous improvement and delivering even better experiences and outcomes for the members who count on us."

Raymond James analysts called ratings declines "relatively modest" at both CVS and UnitedHealth Group, and said the drops at both companies "can be largely mitigated through plan diversification by 2028."

UnitedHealth shares rose 2.25% Friday, to $379.30.