Centene’s stock suffers a historic plunge as underestimation of healthcare costs leads the insurer to withdraw its full-year earnings outlook.

Centene announced it was scrapping its full-year financial forecast after discovering fewer people were signing up for Obamacare health plans, and those that did were less healthy than expected. As a result, CNC was bracing for a $1.8 billion shortfall in risk adjustment revenue.

Shares of Centene cratered Wednesday, a day after the giant managed-care provider withdrew its full-year outlook after receiving new data about enrollments and the health of those covered