Concerns about spiking Covid-19 cases sent US stocks to a negative finish Thursday, overshadowing the optimism from news early in the week about a potential vaccine.

The benchmark Dow Jones Industrial Average lost 1.1 percent to end the session at 29,080.17.

The broader S&P 500 fell 1.0 percent to 3,537.01, while the tech-dominated Nasdaq dropped 0.7 percent to 11,709.59, erasing much of the gains posted Wednesday.

Investors were upbeat after Pfizer and BioNTech on Monday announced promising results from a vaccine that trials showed was 90 percent effective.

But worrying news that Covid-19 cases and hospitalizations were nearing levels seen in the worst days of the pandemic has investors on edge once again.

Wells Fargo analysts said "vaccine optimism is being overshadowed by concerns that renewed restrictions could hinder ongoing economic recovery."

There was little news to counter the jittery sentiment.

New applications for US jobless benefits plunged by 48,000 last week, a far bigger drop than analysts were expecting, taking the level down to 709,000, the Labor Department reported.

Walt Disney, which after the close reported a loss of $710 million in the latest quarter, ended down 1.7 percent.

But the shares jumped 5.5 percent in after-hours trading on news its recently launched streaming television service Disney+ had hit 73 million subscribers.

Pfizer lost 2.5 percent amid the selloff.

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