Dow plummets 726 points for worst day of 2021 as virus variants threaten global recovery

• US stocks fell sharply Monday as investors grew concerned about the global spike in COVID-19 cases.
• The 10-year Treasury note declined as much as 12.2 basis points to 1.177%.
• Cryptocurrencies, gold, and oil were all lower. Lumber prices rose.

US stocks cratered on Monday as investors eyed a spike in global COVID-19 cases led by the Delta variant, which threw up a roadblock to a full recovery of the economy.

The Dow Jones Industrial Average fell 726 points, or about 2.1%, for its worst day since October, while the benchmark S&P 500 and the tech-heavy Nasdaq composite also tumbled.

The yield on the 10-year Treasury note declined as much as 12.2 basis points to 1.177%, its lowest level since February, as investors flocked to safe-haven assets.

Here's where US indexes stood at the 4:00 p.m. ET close on Monday:

• S&P 500: 4,258.49, down 1.59%
• Dow Jones Industrial Average: 33,962.04, down 2.09% (725.81 points)
• Nasdaq composite: 14,274.98, down 1.06%

Read more: 'More weakening beneath the surface': A Wall Street strategist who warned investors before last year's 35% crash lays out the latest signs that another slump into a bear market is looming

"COVID has returned to the front burner of investor concerns right now," David Donabedian, the chief investment officer of CIBC Private Wealth, said in a note. "Last week we had high inflation readings. Now we have concerns that the rise in COVID cases is dimming the economic outlook. While the second-quarter earnings reports have so far beat expectations, this is old news now."

Shares of airlines, cruise operators, and other travel companies slumped on concerns that the Delta variant would derail the recovery.

American Airlines and the airplane maker Boeing all slipped roughly 5% each. Expedia Group and the hotel chain Marriott both declined by roughly 3% each. Meanwhile, Carnival, Norwegian Cruise Line Holdings, and Royal Caribbean Cruises all fell as well.

Energy stocks tumbled, including the Texas oil-equipment maker NOV and Diamondback Energy.

Some said the plunge on Monday was nothing to fear. The sell-off in stocks is a "healthy pullback" that will likely be short-lived and could present a buying opportunity, Katie Stockton, a technical analyst at Fairlead Strategies, said.

In cryptocurrencies, bitcoin continued its recent slide, falling as much as 3.4% to $30,646.90. All other major cryptocurrencies - ether, cardano, ripple, dogecoin, polkadot, and solana - traded lower on Monday.

Despite the downturn, mining bitcoin has been a lot easier. The asset's "network difficulty," which measures how much computing power is needed to mint a new bitcoin, has plummeted.

Oil fell on news over the weekend that OPEC+ reached a deal on supply to overcome the deadlock between Saudi Arabia and the United Arab Emirates.

West Texas Intermediate crude fell as much as 8.06% to $66.02 a barrel. Brent crude, oil's international benchmark, dropped 7.39% to $68.15 a barrel at intraday lows.

Gold fell as much as 0.45% to $1,807.56 an ounce.

Lumber gained modestly, rising 4.83% to $561.90 as supply was catching up with demand. Prices are set to stay elevated despite recent declines, a Fastmarkets economist said.