Investors around the globe are looking to Monday’s early trading for clues to the scale of the threat posed by the emergence of a new Covid-19 variant.

Though occasional sharp reversals have become commonplace during the bull market that started last year after the lockdown-inspired spring market rout, traders and portfolio managers were struck by the ferocity of selling on Friday. U.S. stock trading volume hit the highest level on the day after Thanksgiving in at least 14 years, with the Dow Jones Industrial Average and S&P...

Investors around the globe are looking to Monday’s early trading for clues to the scale of the threat posed by the emergence of a new Covid-19 variant.

Though occasional sharp reversals have become commonplace during the bull market that started last year after the lockdown-inspired spring market rout, traders and portfolio managers were struck by the ferocity of selling on Friday. U.S. stock trading volume hit the highest level on the day after Thanksgiving in at least 14 years, with the Dow Jones Industrial Average and S&P 500 posting their largest-ever Black Friday declines, falling 2.5% and 2.3%, respectively.

While half-staffed trading desks on Black Friday may have resulted in exaggerated market moves, some traders and portfolio managers said they believe it may be some time before markets can fully assess the risk of fresh Covid-related economic disruption.

““There’s a lot of questions and markets don’t like uncertainty.””

— Larry Milstein, R.W. Pressprich & Co.

On Monday morning in Asia, U.S. stock-index futures rose, suggesting U.S. markets could open higher later in the day. E-mini S&P futures were up 0.8%, while futures tied to the Dow were 0.6% higher. Oil prices also rebounded partially in Monday morning trading, with U.S. crude oil jumping 5% to $71.52 a barrel after tumbling 13% last Friday.

Stock markets in Asia were broadly lower in early trading. Japan’s Nikkei 225 index fell 0.3%, extending losses from Friday, and Australia’s S&P/ASX 200 dropped 0.4%. Australia is among the countries that recently detected cases of the Omicron variant and that have tightened border controls again. Indexes in China and Hong Kong also fell.

While some countries have moved quickly to manage the emerging risk from the new variant, it could be weeks before Omicron’s actual transmissibility and severity is known, said Alvin Tan, head of Asia FX Strategy at RBC Capital Markets in Hong Kong. “The markets have already priced in a strong recovery, so this element of uncertainty is negative. But it is unlikely to be a repeat of the beginning of the pandemic,” he said.

Investors will remain risk-averse until they receive more information about the new Covid-19 variant, said Larry Milstein, head of treasury and agency trading at R.W. Pressprich & Co. He expects to continue seeing investors move into safer assets like government bonds at the start of the week. The 10-year U.S. Treasury yield dropped to 1.484% Friday from 1.644% on Wednesday, reflecting a jump in prices.

“There’s a lot of questions and markets don’t like uncertainty,” he said.

Friday’s market moves in part reflect pushback against the widespread confidence in the economic recovery that had propelled benchmark bond yields higher in recent weeks and supported the rise in stocks.

“When everybody is kind of leaning to one side of the boat, it doesn’t take much to get everyone to go back in the other direction,” said John Kerschner, head of U.S. securitized products at Janus Henderson Investors.

While the emergence of the recent Covid-19 variant is “definitely scary,” Mr. Kerschner said, the market swoon may have been an overreaction that could reverse when more traders are back in the office.

On Black Friday, there were fewer people at their desks, said Mr. Kerschner. “Whenever it happens on a low volume day, it seems to get out of proportion...We’ll see how things look on Monday,” he added.

Though the Covid-19 news bears scrutiny, investors and analysts said, for now there is no reason to doubt the combination of economic expansion and low interest rates that has brought major U.S. stock indexes to 156 records this year.

Jim Vogel, interest-rate strategist at FHN Financial, said investors still expect the Federal Reserve to raise interest rates twice in 2022.

For now, investors are taking precautions by selling in what had become an expensive equity market, said Christopher Sullivan,

chief investment officer at United Nations Federal Credit Union. As some investors bought Treasurys, those who had borrowed Treasurys and sold them in a bet their value would decline were forced to buy them back to cover, he added.

The scale of Friday’s selloff in stocks and oil and the rally in Treasurys “seems something of an overreaction to my mind, given the many unanswered questions about the new variant that has taken hold in South Africa,” said Mr. Sullivan.

At the same time, much will depend on how assets such as stocks and industrial commodities trade on Monday, when people return to their desks from the holiday weekend.

“There’s going to be a lot of people that this catches by surprise, so you can expect to see some follow through on Monday morning,” said Mr. Vogel of FHN.

After scientists identified a new variant of the virus causing Covid-19, countries restricted travel to and from southern Africa. WSJ’s Anna Hirtenstein explains that investors have turned to bonds and gold as they prepare for more potential disruption. Photo: Sumaya Hisham/Reuters The Wall Street Journal Interactive Edition

—Serena Ng contributed to this article.

Write to Sebastian Pellejero at sebastian.pellejero@wsj.com and Ben Eisen at ben.eisen@wsj.com