Stocks and oil are whipped around by the Fed and Omicron’s arrival in the U.S.

Stocks on Wall Street were unsteady on Wednesday, continuing their tumultuous ride since the discovery of the new Covid-19 variant last week.

The S&P 500 was up just 0.2 percent by midafternoon, as an early gain of more than 1 percent quickly faded following news that the Omicron variant of the coronavirus had been detected in the United States. The Nasdaq composite, which had been positive territory, turned negative as the infection was announced.

Early gains by oil futures also faded. West Texas Intermediate, the U.S. benchmark, was down slightly by midafternoon after earlier rising as much as 5 percent.

Shares of companies likely to be most affected by an increase in pandemic precautions were among the hardest hit. American Airlines fell more than 7 percent and was one of the worst performers in the S&P 500. United Airlines was down about 6 percent, and the cruise lines Norwegian and Carnival were down more than 4 percent.

As they consider the risk of a new variant of the coronavirus, and the potential impact on the global economy as governments once again restrict travel and tighten testing requirements, investors are also grappling with a shifting outlook for interest rates. On Tuesday, the S&P 500, the U.S. benchmark, declined 1.9 percent when the head of the Federal Reserve said that the central bank might speed up its plan to reduce support for the economy because of high inflation.

A measure of volatility in the U.S. stock market surged to its highest since early March on Friday after the new Omicron variant was reported by researchers in South Africa. The VIX index has declined a little since then, but it remains above levels seen in the past two months.

Traders had pushed back their expectations about when the Fed might eventually raise interest rates, in light of the news about the variant and some predictions that current vaccines will be less effective against it. But Jerome H. Powell, the Fed chair, said on Tuesday that the risk of higher inflation had increased. If the central bank finishes tapering its bond-buying program sooner than expected, it could also raise interest rates sooner.

The Omicron variant could prolong the bottlenecks and shortages that have caused inflation to run hotter than expected, a risk Fed officials will assess as they “grapple” with how quickly to remove economic support, another Fed official said.

“Clearly, it adds a lot of uncertainty to the outlook,” John C. Williams, president of the Federal Reserve Bank of New York, said in an interview The New York Times that was published on Wednesday.