(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 over 15% gain on stimulus, vaccine support * Weekly jobless claims fall for second straight week * Indexes: Dow up 0.03%, S&P up 0.10%, Nasdaq down 0.04% (Updates to open) Dec 31 (Reuters) - U.S. stocks were flat in thin trading on Thursday, but the S&P 500 and the Nasdaq were set to end 2020 with solid gains on hopes of more stimulus and coronavirus vaccines to support the domestic economy. The 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. While the virus continues to take its toll on the economy, unprecedented stimulus from the central bank and the government have helped the benchmark S&P 500 index climb almost 70% from its March low, putting it on course for a more than 15% gain this year. A U.S. Labor Department report showed the number of Americans filing claims for jobless benefits fell for the second straight week, but remain elevated more than nine months into the economic crisis. Six of the major S&P 500 sub-sectors edged lower in early trading, with energy, industrials and materials falling after recent gains. The Nasdaq was set to record a 43% jump in what could be 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. A favorable U.S. election outcome and optimism around vaccines rollouts sparked a rally in risky assets in November, but the momentum stalled on worries over fresh fiscal stimulus and a new, highly infectious COVID-19 variant spreading globally. “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.” 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. 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 10:04 a.m. ET, the Dow Jones Industrial Average was up 8.54 points, or 0.03%, at 30,418.10, the S&P 500 was up 3.61 points, or 0.10%, at 3,735.65, and the Nasdaq Composite was down 5.69 points, or 0.04%, at 12,864.31. Trading volumes are likely to remain light on New Year’s Eve and the markets are closed on Friday. Shares of Tribune Publishing Co rose 9.4% after its largest shareholder, Alden Global Capital, offered to take full control of the owner of the Chicago Tribune in a deal that values the newspaper chain at $520.6 million. Travel-related stocks were among the biggest decliners as investors locked in profits after a recent rally. The S&P 1500 airlines index fell 1.3%, while cruise operators Norwegian Cruise Line Holdings Ltd, Carnival Corp and Royal Caribbean Cruises Ltd slipped between 3.9% and 2.3%. Declining issues outnumbered advancers for a 1.49-to-1 ratio on the NYSE and a 2.19-to-1 ratio on the Nasdaq. The S&P index recorded four new 52-week highs and no new lows, while the Nasdaq recorded 55 new highs and nine new lows. (Reporting by Devik Jain and Supriya R in Bengaluru; Editing by Sriraj Kalluvila and Arun Koyyur)