Major health insurers' shares sold off in morning trading Tuesday as Humana (HUM) said Medicare Advantage reimbursement rates will at a "mid-single-digit" pace vs. its prior forecast for "flat to slightly down." Humana, which has some 2 million Medicare Advantage members, fell more than 7% intraday in the stock market today. UnitedHealth (UNH) lost 2%, Aetna (AET) and Cigna (CI) fell 1.5% and WellPoint (WLP) retreated less than 1%. The Centers for Medicare