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Short sellers are suffering since Donald Trump won

The "experts" weren't the only ones surprised by Donald Trump's victory.

It seems that short sellers — people who bet that individual stocks were going to fall — also were caught unaware by the electoral upset on Nov. 8.

The companies with the highest levels of short interest prior to the election have rallied the most in the days since, according to data from research firm Markit. The majority of stocks on the Russell 3000 have seen gains in the past two weeks, and those gains have been concentrated in the stocks that short sellers were betting would drop following the election.

The 300-some companies that had the most shares outstanding on loan — a common measure of short selling — have seen average returns of 12.8 percent as of the close Monday. That's compared to 8.6 percent for the tenth of companies with the fewest shares on loan.

Prime areas of pain for short sellers are health care and biotech stocks, according to research analyst Simon Colvin. Short sellers had acted on the assumption that a Clinton win would bring increased regulation to health care companies, a fear that evaporated when Trump won.

"Short sellers' pain was also compounded as their favorite biotech firms surged by 22% in the last week, more than the 16% advance seen by the wider Russell 3000 biotech sector," Colvin wrote in a note.

Consumer companies with strong short selling interest also have risen in the last two weeks: clothier Lands End, Shake Shack and Tootsie Roll among them.

Not shorting the rally

But short sellers aren't interested in shorting the current "Trump bump," which has pushed all three indexes to all-time highs on Tuesday. The Dow Jones industrial average hit levels above 19,000 for the first time on Tuesday, and history suggests it could keep climbing.

According to Markit's data, the most-shorted group of stocks saw a 1.5 percent drop in average demand to borrow in the week following the election.