Exchange-traded fund investors are throwing caution to the wind, betting the S&P 500’s record run isn’t stopping anytime soon.

Short interest as a percentage of shares outstanding on the SPDR S&P 500 ETF Trust -- a rough indicator of bearish bets on U.S. stocks -- fell to as low as 2.4% this week, according to data from IHS Markit Ltd. That’s the smallest percentage since October 2018, just before U.S. stocks suffered the bulk of their fourth-quarter downturn.

Optimism is increasing among fund investors after the S&P 500 surged 11% in less than three months, bringing its total return this year to 30%. In a December analysis of institutional money managers from RBC Capital Markets, those describing themselves as “bearish” dropped to 15%, the lowest level since the third quarter of 2018.

In another sign of confidence, flows into stock ETFs have surged to $75 billion this quarter, more than double the amount that’s entered bond funds. In the first nine months of 2019, flows into fixed-income ETFs were higher.

But some are beginning to worry about the euphoria.

The SPDR S&P 500 ETF Trust suffered its biggest one-day outflow since September on Thursday, with investors yanking $5 billion from the $295 billion fund.

“There are good reasons to be optimistic, but at the same time the best course of action is to stay cautious, especially when the markets are near the highs like they are,” Chris Gaffney, president of world markets at TIAA, said by phone. “When all the investors get on one side of the boat, it’s always cause for concern.”

This article was provided by Bloomberg News.