Shares of UnitedHealth Group surged nearly 14% on Friday after billionaire Warren Buffett's Berkshire Hathaway bought 5 million shares of the company, providing a shot in the arm for investors who think the health conglomerate will turn around under its new CEO.

The Oracle of Omaha just took a $1.6 billion stake in the battered healthcare giant, sending shares soaring 9% after hours.

UnitedHealth shares clocked the largest one-day percent increase since March 2020, as tracked by Dow Jones Market Data Group. It was on pace for the best day in 17 years before giving back some of those gains. For the week, shares rose over 21%, the best week since May 2009. Still, the stock is down more than 50% over the past 12 months.