The Dow Jones Industrial Average fell more than 150 points on Thursday, the 30-stock average's fourth day of losses.
The blue-chip index sank 151.69 points to 34,879.38, dragged down by Amgen and Merck.
The S&P 500 docked 20.79 points to 4,493.28
The NASDAQ Composite dropped 38.38 points, to 15,248.25.
All three major averages are headed for losses this week.
Despite recent losses, the major benchmarks are still up big for the year and within striking distances of their all-time highs. The S&P 500, up roughly 20% in 2021, is about 1.2% from its all-time high. The Dow, up 14% this year, is about 2% from a record.
Several airlines on Thursday lowered their forecasts because of the resurgence in COVID. United Airlines, American Airlines and Southwest Airlines each gave cautious comments; however, their stocks remained in the green.
Boston Beer fell 3.8% after pulling its earnings guidance amid slowing growth in its hard seltzer brand.
Meme-favorite GameStop closed in the green after dropping as much as 10.5%. The video-game retailer posted a narrower loss compared with the year prior but did not provide an outlook or grander turnaround plans.
Moderna shares rose more than 7.8% after the drug maker said it's developing a single dose vaccine that combines boosters against Covid and the flu.
Shares of athletic retailer Lululemon surged 10.5% and furniture retailer RH rose 7.8% on the back of better-than-expected earnings.
Lululemon also offered a stronger-than-forecast outlook for the third quarter and the year.
Investors remained cautious as they try to discern what's next to happen with the delta variant, the economic reopening and the Federal Reserve.
The Fed meets on Sep. 21-22 and investors are worried the central bank will indicate a move to slow down its monthly $120 billion in bond purchases, which have kept rates low and boosted the recovery from the pandemic.
Also helping sentiment was a better-than-expected weekly reading on jobless claims. Initial jobless claims came in 310,000, which was below expectations of 335,000 claims. This marked another fresh low for the pandemic era.
Prices for 10-Year Treasurys gained ground, lowering yields to 1.30% from Wednesday's 1.34%. Treasury prices and yields move in opposite directions.
Oil prices withered $1.32 to $67.98 U.S. a barrel.
Gold prices surged $3.90 to $1,797.40 U.S. an ounce.