U.S. airline shares had gained ground since November’s vaccine breakthroughs but came under renewed pressure earlier this week amid concern over new coronavirus variants and tighter restrictions. American reported a net loss of $2.18 billion, or $3.81 per share, for the fourth quarter, compared with a profit of $414 million, or 95 cents per share, a year earlier. The carrier reported an annual loss of $8.9 billion, its biggest on record. Shares climbed 26% to $21 a share in early trading even as the heavily leveraged airline reported large losses and faces an uncertain industry outlook. Investors said the gains are likely fueled by a short squeeze. American is the most heavily shorted airline stock, according to data from S3.