The Greek alphabet isn't gaining any fans, with "omicron" now a dirty word for investors. The Dow Jones Industrial Average (DJINDICES:^DJI) fell 461.7 points, or 1.3%, to kick off December. The biggest culprit was investors' reaction broadly to the latest strain of the coronavirus. The first U.S. infection caused by the variant was identified in California, and cases in South Africa, where omicron is thought to have originated, doubled overnight.

Today's sell-off was broad across the stock market and within the Dow components. More than half of stocks fell, with aircraft giant Boeing (NYSE:BA) and chemical maker Dow (NYSE:DOW) both losing more than 3%, while retailers Walmart (NYSE:WMT) and Walgreens Boots Alliance (NASDAQ:WBA) both lost 2%. A wide swath of companies that sell consumer goods fell 1% or more, while healthcare giant and coronavirus vaccine maker Johnson & Johnson (NYSE:JNJ) was the Dow's biggest winner, up 1.7% today. 

The biggest loser in the Dow Jones today was one of its newest components, Salesforce.com (NYSE:CRM), down 11% following its third-quarter earnings report, released after the bell on Nov. 30. 

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Salesforce.com and messed-up expectations

The software giant reported third-quarter results after hours yesterday, with $6.9 billion in revenue and $1.27 billion adjusted earnings per share, beating investors' expectations soundly. But when the company rolled out its guidance for the upcoming quarter and beyond, the stock lost its luster, shedding more than 11% of its value

How bad is it expected to be? Management put revenue expectations at about $7.22 billion at the midpoint, and adjusted earnings per share of $0.73, while Wall Street analysts were looking for $0.82 per share. Ouch. 

Coronavirus variant rattling investors

Today's sell-off was broad, with investors worried about the economic implications of travel restrictions, people avoiding public places, and a surge in cases if the omicron variant is less affected by vaccines. Shares of almost every Dow stock that makes consumer products, provides financial services, or operates businesses open to the public fell today. The main exceptions were Home Depot (NYSE:HD), which is likely to continue to see strength as people concentrate on their homes, and consumer staples producers like Coca-Cola (NYSE:KO) and Procter & Gamble (NYSE:PG), and drive-thru giant McDonald's (NYSE:MCD); but even these were up modestly, as worry won the day. 

Keeping the long view

The Dow Jones Industrial Average is now down 6.6% since the all-time high on Nov. 8, with investor sentiment leaning more toward worry -- and now with omicron, fear -- as we enter the final month of 2021. Whether stocks continue to fall and enter a full-on correction or bear market, or revert back to bull territory remains to be seen. Whatever happens over the next few weeks and months, though, days like today are a good reminder that investing in stocks is most successful when it's done over many years, not just a few weeks or even months. 

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.