The S&P 500 has ended lower after the launch of a COVID-19 vaccine campaign in the United States, while Alexion Pharmaceuticals has jumped on a $US39 billion ($A52 billion) buyout offer from AstraZeneca.

The Dow Jones Industrial Average hit a record high before ending lower, weighed down by Walt Disney.

Administration of the vaccine developed by Pfizer and its German partner BioNTech began on Monday following emergency-use approval from federal regulators last week.

The S&P 500 consumer discretionary index was one of the strongest gainers among sector indexes, lifted by a rise in Amazon. The energy index tumbled.

The S&P 500 gave up earlier gains of almost 1 per cent. The index has surged about 13 per cent to record highs in 2020, despite the pandemic, which has wrought economic devastation and killed more than a million people.

“While the entire market is pleased, is optimistic, is bullish about the arrival of the vaccine this morning … I think the average investor is realising that this roll-out, this distribution of the vaccine is not going to be a silver bullet, is not going to go as fast as one hopes,” said Jake Dollarhide at Longbow Asset Management in Tulsa.

Alexion Pharmaceuticals Inc was among the top boosts to the S&P 500 and the Nasdaq, surging to a 4-1/2 year high after British drugmaker AstraZeneca said it would buy the US biotech firm.

AstraZeneca’s US-listed shares dropped.

Walt Disney fell after BMO Capital Markets downgraded the stock.

Unofficially, the Dow Jones Industrial Average fell 0.62 per cent to end at 29,859.97 points, while the S&P 500 lost 0.44 per cent to 3,647.33.

The Nasdaq Composite climbed 0.5 per cent to 12,440.04.

Investors were also watching the outlook for additional coronavirus relief after the Senate last week approved a one-week extension of funding to avoid a government shutdown and allow more time for negotiations.

Investors were also focused on early voting in a pair of US Senate races in Georgia that will determine control of the chamber and heavily influence lawmaking.

E-commerce company Alibaba Group Holding Ltd dipped after China warned its internet majors of more anti-trust scrutiny, imposed fines and announced probes into deals involving Alibaba and Tencent Holdings Ltd.

Electric-car maker Tesla Inc rallied as anticipation of its addition to the S&P 500 benchmark next week offset a report of production delays.