On Wednesday, the S&P 500 was lifted by gains in health care, technology and consumer-oriented stocks.
Biogen jumped 9.5% for the biggest gain in the index. Target, Nike, supermarket operator Kroger and auto products retailer AutoZone rose 1.3% or more.
Facebook parent Meta Platforms slipped 0.9%, Exxon Mobil dropped 0.9% and Morgan Stanley fell 1.2%.
Trading was lighter as investors closed out positions for the year. Fewer than 3 billion shares traded hands on the New York Stock Exchange compared with a typical daily average of 4.5 billion.
Investor concerns about the omicron variant eased after researchers said it appears to cause less severe symptoms and President Joe Biden avoided announcing travel or other restrictions that might weigh on economic activity.
Still, markets are uncertain about the impact of omicron, which is spreading fast and quickly becoming the dominant variant.
In energy markets, benchmark U.S. crude rose 38 cents to $76.95 per barrel in electronic trading on the New York Mercantile Exchange. The contract advanced 58 cents to $76.56 on Wednesday. Brent crude, the price basis for international oils, gained 38 cents to $79.59 per barrel in London. It closed 29 cents higher the previous session at $79.23.