Short sellers have revived their wagers against the stock market in recent weeks, taking their most aggressive positions in years.

Bets against the SPDR S&P 500 Trust, the biggest exchange-traded fund tracking the broad index, rose to $68.1 billion last week, the highest level in data going back to January 2016, according to financial analytics company S3 Partners. That was up from $41.7 billion at the beginning of 2020 and $41.2 billion a year ago.

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