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Warren Buffett dumped the 'big 4' airline stocks in April. They've already recovered from the sell-off he sparked.

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Warren Buffett revealed at the start of May that he'd dumped the "big four" airlines in April, sending their stocks into freefall. They've bounced back in less than a month as investors bet the "Oracle of Omaha" made a bad call.

Shares in American Airlines, Delta Air Lines, Southwest Airlines, and United Airlines jumped last week on coronavirus vaccine hopes, then soared more than 12% on Tuesday after a raft of positive news. All four were up more than 7% in pre-market trading as of 7:30 a.m. ET on Wednesday.

The stocks have now erased their losses since Buffett announced his sales, and trade higher than their average price in April, meaning they could be worth more now than when Buffett sold them.

It's worth revisiting why Buffett ditched the "big four" airlines, and what has happened since then to justify the recent rally.

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'The world has changed'

Buffett's Berkshire Hathaway conglomerate owned between 8% and 11% of the "big four" airlines before it ditched those positions in April. The billionaire investor announced the sales and explained them at Berkshire's shareholder meeting earlier this month.

His main argument was that the investment case for the airlines has changed significantly. He questioned whether passenger numbers would fully rebound from the coronavirus pandemic in the next few years. Even if carriers returned to 80% of their capacity, he warned, they could be left with "too many planes."

Buffett also pointed to the billions of dollars in government loans taken by the carriers, which they will have to repay out of their earnings. He added that shareholders will also lose out from the the stock warrants that the airlines handed to the government, which it can exercise to buy their shares at a discount in the future.

"The world has changed for the airlines," Buffett said at the Berkshire meeting. "The future is much less clear to me about how the business will turn out."

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Betting on recovery

Investors shrugged off Buffett's concerns on Tuesday, adding $6 billion to the "big four" airlines' market capitalizations.

Early signs of passenger numbers rebounding, travel restrictions being eased, flights resuming, and carriers adapting to the pandemic may explain their renewed optimism:

While those developments are certainly positive, it's too soon to say whether passenger numbers will fully recover, and Buffett's warnings about loan repayments and shareholder dilution still ring true.

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Here's a chart showing how the "big four" airline stocks have rebounded since Buffett announced his exit:

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