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Investors celebrated the biggest government intervention in the U.S. economy with the first back-to-back days of stock gains in more than a month. But with the coronavirus outbreak unquantified and untamed, many analysts say it’s too soon to declare an end to the weeks-long market slump.

The stock rally follows trillions of dollars in commitments this week from the Federal Reserve and Capitol Hill designed to prevent the economy from descending into prolonged stagnation. Big, rapid swings in market indexes, however, indicate Wall Street is looking for more than just federal dollars.

“The two-day rally we are experiencing does not constitute a bull market,” said Terry Sandven, chief equity strategist at U.S. Bank. “Monetary assistance is helping the broad market and we are getting some assistance from Congress. That’s all positive. But the duration and impact of covid-19 remain unknown. Until we get greater clarity, we expect volatility is going to remain high.”

The Dow Jones industrial average climbed 495.64 points, or 2.4 percent, to close Wednesday at 21,200.55 as investors look to the $2 trillion rescue package to filter into every corner of American business. The Dow was coasting to a more than a 1,000-point gain with minutes left in the session when word of a possible delay in the Senate erased hundreds of points.

The chamber voted on the package late Wednesday after its leaders worked out some last-minute snags. New York Gov. Andrew M. Cuomo (D) demanded changes to help his state respond to a flood of new cases. Three Republican senators said a provision in the bill needed to be fixed immediately or it would incentivize people not to return to work. And House Democrats wouldn’t provide a firm timeline of when they would vote on the bill.

Wednesday’s stock gains built on Tuesday’s monster rally, which powered the Dow to its best finish in 87 years. It also capped the blue chips’ first back-to-back advances since early February. And though the Dow has climbed 14 percent in two days, it is 26 percent in the hole for 2020 and still on track for its worst month since the October 1987 crash.

The broader Standard & Poor’s 500 index jumped 1.2 percent, to 2,475.56, while the Nasdaq composite shed 0.5 percent to end the day at 7,384.30.

Energy, real estate and industrials were the leading sectors, but airlines also surged on reports that the coronavirus rescue bill contains significant aid for the industry.

Boeing was the big winner of the day — responsible for about a third of the Dow’s gain, according to Howard Silverblatt of S&P Dow Jones Indices. Reports that the Chicago-based aerospace giant could be in line for billions under the stimulus package sent its stock soaring 24.3 percent. It was trading below $100 on Monday. It closed Wednesday at $158.73.

Senate leaders and the White House came to an agreement on a $2 trillion stimulus package early Wednesday. The largest bailout in U.S. history would mean $1,200 checks for many Americans and include a $367 billion loan program for small businesses and a $500 billion fund for industries, cities and states. It also covers $150 billion for state and local stimulus funds, $130 billion for hospitals and additional assistance for unemployment insurance.

Investors are happier but cautious.

“We view this package as a turning point in this crisis because it also shows a political willingness to do more later on if needed,” wrote Torsten Slok, chief economist at Deutsche Bank Securities, in an email to The Washington Post. “This package will flatten the recession curve and we now need to see a flattening of the virus curve.”

Five days of negotiations had sent markets in all directions — stocks tanked Monday after the Senate twice failed to advance the coronavirus stimulus bill, then surged as a deal appeared imminent. On Tuesday, Wall Street staged a massive rally in anticipation of an agreement, with the Dow climbing 11.4 percent for its best day since 1933. The S&P 500 Index jumped 9.4 percent and the tech-heavy Nasdaq climbed 8.1 percent.

The U.S. economy still must overcome a slew of hurdles after the bill’s passage. Lauren Goodwin, economist and portfolio strategist at New York Life Investments, said in a note that looming questions remain about how to save jobs already lost and how quickly the government can send cash straight to households and businesses in urgent need.

“How much will be ‘wasted’ on companies that don’t need it as much, such as those with larger cash reserves or online sales?” Goodwin said. “What ‘unknown unknowns’ lie ahead in terms of implementation, which could create market stress?”

Ed Yardeni, president of Yardeni Research, said the country needs to get the virus under control before the economy can breathe on its own, especially as numerous industries have shut down.

“This is a world war against the virus,” Yardeni said. “While we spend money to soften the blow on workers and companies, we need to mobilize resources to fight the war. There is a desperate need for millions of masks, ventilators and a Manhattan Project for finding a cure.”

The Manhattan Project was the secret and massive undertaking by the United States during World War II that resulted in the creation of the atomic bomb.

In Europe, Britain’s FTSE 100 closed up 4.4 percent, and Germany’s DAX grew 1.8 percent. The benchmark Stoxx 600 jumped 3.1 percent. The coronavirus crisis intensifies by the day on the continent: The death toll in Spain now stands at 3,400, surpassing China’s, making it the hardest-hit country behind Italy. Prince Charles, heir to the British throne, has tested positive for the coronavirus.

Asian markets flashed green across the board. The Nikkei 225 soared 8 percent, even after the Tokyo Games were postponed until 2021. Hong Kong’s Hang Seng advanced 3.8 percent. The Shanghai composite popped 2.2 percent.

Erica Werner, Mike DeBonis and Paul Kane contributed to this report.