(For a Reuters live blog on U.S., UK and European stock markets, click LIVE/ or type LIVE/ in a news window.) * S&P 500 set for more than 15% gain on stimulus, vaccine support * Weekly jobless claims fall for second straight week * Futures: Dow off 0.03%, S&P down 0.05%, Nasdaq up 0.05% (Adds comment; updates share prices) Dec 31 (Reuters) - U.S. stocks were set for a muted open on the last trading session of a tumultuous year on Thursday, as investors digested big market gains and hoped that more stimulus and coronavirus vaccines will drive a strong economic recovery in 2021. Wall Street’s three main indexes surged to record highs this week in a stunning recovery since March when the COVID-19 pandemic triggered the steepest global recession in generations and left millions of Americans unemployed. Unprecedented levels of central bank and government stimulus have helped the benchmark S&P 500 index climb almost 70% from its March low and put it on course for a more than 15% annual gain. The Nasdaq is set to record a 43% jump, its best yearly performance since 2009, benefiting from a surge in tech mega-caps such as Alphabet Inc, Microsoft Corp , Apple Inc, Facebook Inc and Netflix Inc. Still, major world economies are grappling with a jump in cases, with a highly infectious COVID-19 variant - originally discovered in Britain - detected in California, a day after the first known U.S. case was documented in Colorado. “Vaccine versus accelerating cases... that will be the story of the market for the first couple of months,” said Christopher Grisanti, chief equity strategist at MAI Capital Management. “The vaccine will win. The market remains strong in the first half of 2021.” A favorable U.S. election outcome, large-scale vaccination campaigns, steady improvement in economic data and signs that a fiscal stimulus deal will be eventually reached in Washington helped Wall Street continue its rally. Near-term expectations of bigger stimulus checks dimmed after Senate Majority Leader Mitch McConnell blocked a quick vote on Wednesday to back President Donald Trump’s call to increase COVID-19 relief checks to $2,000 from $600. Meanwhile, a U.S. Labor Department report showed the number of Americans filing first-time claims for unemployment benefits fell for the second straight week, but remain elevated more than nine months into the crisis triggered by the pandemic. All eyes are on two U.S. Senate races in Georgia next week that will determine control of the chamber and influence Democratic President-elect Joe Biden’s ability to enact his agenda. At 08:49 a.m. ET, Dow e-minis were down 10 points, or 0.03%, S&P 500 e-minis were down 2 points, or 0.05%, and Nasdaq 100 e-minis were up 6.5 points, or 0.05%. Trading volumes are expected to be light with many traders away on New Year’s Eve.