Despite a rollout of Covid-19 vaccines beginning sooner than many expected, large institutional investors don’t think the stock market can extend its rally into next year.

A Natixis Investment Managers survey of 500 institutional investors who manage more than $13.5 trillion in assets found that more than three-quarters of respondents think the stock market’s current pace of growth is unsustainable. As a result, many are positioning portfolios defensively in 2021, despite the broad rebound this year.

“Investors’ outlook reflects deep concerns about the lasting consequences of the extreme measures needed to cushion the financial blow of the pandemic,” said David Giunta, CEO for the U.S. at Natixis Investment Managers, in a release.

“However, they also see opportunities to find value through active management, thoughtful portfolio allocation and diversification.”

The survey found broad asset allocations aren’t changing much, but institutional investors are making tactical shifts within asset classes.

Within equities, the survey found, many investors are planning to trim U.S. holdings and increase exposure to European, Asia Pacific and emerging market stocks.

Investment grade corporate debt and securitized loans are more popular on the fixed income side, at the expense of government bonds, the report said.

Investors are also increasing exposure to alternative strategies such as private equity and infrastructure. More than two-thirds said private assets would play a more prominent role in their portfolios going forward.

While growth stocks surged this year during pandemic lockdowns, more than half of those surveyed (58%) said they expect value to outperform growth in 2021. A narrow majority are betting on large-cap names over small-cap names, and on emerging markets outperforming developed markets.

However, almost three-quarters of respondents expect large technology companies to continue to thrive even in the face of potential regulatory action.

The top market risks for institutional investors include negative rates and volatility in equities and currency markets. More than eight in 10 said low rates have distorted market valuations, and more than seven in 10 said institutional investors are taking on too much risk in their search for yield.

The upside of volatility is the opportunity to outperform, and two-thirds of respondents expected active investors would beat benchmarks.

Research firm CoreData conducted the survey for Natixis in October and November. Download the full report here.