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* Alibaba sinks on Chinese regulatory ire

* Amazon leads gains in consumer discretionary

* Alexion Pharma among top boosts on AstraZeneca offer (Updates with closing prices)

By Karen Pierog and Noel Randewich

Dec 14 (Reuters) - The S&P 500 ended lower on Monday, weighed down by Walt Disney, while Alexion Pharmaceuticals jumped on a $39 billion buyout offer from AstraZeneca in one of the year's biggest deals.

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

U.S. officials began to administer the vaccine developed by Pfizer and its German partner BioNTech on Monday following emergency-use approval from federal regulators last week.

Shares of Disney, down almost 4%, and Pfizer, down almost 5%, weighed more than any other stocks on the S&P 500.

The index's four-day losing streak was its longest since Sept. 21.

The S&P 500 consumer discretionary index was the strongest gainer among sector indexes, up 0.5% and lifted by a rise in Amazon, up 1.3%. The energy index tumbled over 3%.

The S&P 500 gave up earlier gains of almost 1%. The index has surged about 13% 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 into the U.S., I think the average investor is realizing 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, chief executive officer of Longbow Asset Management in Tulsa, Oklahoma.

Alexion Pharmaceuticals Inc was among the top boosts to the S&P 500 and the Nasdaq, surging about 30% to a 4-1/2 year high after British drugmaker AstraZeneca said it would buy the U.S. biotech firm. AstraZeneca's U.S.-listed shares dropped almost 8%.

Walt Disney fell after BMO Capital Markets downgraded the stock following its recent gains and said Netflix was again its "top pick." Netflix climbed almost 4%.

Unofficially, the Dow Jones Industrial Average fell 0.62% to end at 29,861.55 points, while the S&P 500 lost 0.44% to 3,647.49. The Nasdaq Composite climbed 0.50% to 12,440.04.

Also on Monday, negotiators in the U.S. Congress neared agreement on a massive government spending deal that would avert a government shutdown, as Republicans and Democrats insisted they want to pass a fresh round of aid to a coronavirus-hit nation.