The Dow Jones Industrial Average posted its best daily gain since early March as investors turned more bullish on Monday, following last week's volatile stretch that had been sparked by concerns about the spread of the omicron variant of the coronavirus that causes COVID-19. Fears of a policy error by the Federal Reserve also have been making investors uneasy. On Monday, the Dow /zigman2/quotes/210598065/realtime DJIA -0.38% closed up 1.9% or about 647 points, to reach 35,227, powered by gains in UnitedHealth Group Inc. /zigman2/quotes/210453738/composite UNH +0.25% and Goldman Sachs Group Inc. /zigman2/quotes/209237603/composite GS +0.13% , while the S&P 500 index /zigman2/quotes/210599714/realtime SPX -1.04% closed up 1.2% to reach 4,591 and the Nasdaq Composite Index /zigman2/quotes/210598365/realtime COMP -1.59% advanced 0.9% to end at around 15,225. The more upbeat trading in stocks came as Dr. Anthony Fauci, the U.S.'s leading specialist in infectious diseases, offered encouraging comments on the outlook of the omicron variant over the weekend. Fauci told CNN's "State of the Union" that early reports about the spread of the omicron variant of coronavirus suggest it might be less severe than initially feared. Shares of vaccine maker Moderna /zigman2/quotes/205619834/composite MRNA -1.76% , meanwhile, ended down more than 13%.