One of the country's largest providers of private Medicare plans saw its stock sink to its lowest level in 15 years after the federal government cut the rating for one of its most popular offerings. Shares of Humana plunged Wednesday after the company revealed in a filing to the SEC that only 1.6 million people — about a quarter of its members — are signed up for Medicare Advantage plans rated four stars or higher for 2025. This year, 94% of its members are enrolled in them.