(Reuters) - Shares of Tesla dropped in their S&P 500 debut on Monday, pulling back from record-highs as worries over a fast-spreading new coronavirus strain in Britain weighed on markets.Tesla"s stock sank about 4% to $665 at mid-day in its first session in the S&P 500 .SPX. The shares were the biggest drag on the S&P 500, accounting for 0.07 percentage point of the index"s 0.46% decline, according to Refinitiv data. Wall Street was down broadly after a more-virulent strain of the coronavirus in Britain sparked fears of fresh disruptions. The electric car maker, headed by billionaire Elon Musk, becomes the most valuable ever admitted to Wall Street"s main benchmark and accounted for a 1.69% weight in the index ahead of Monday"s trading. The shares had surged some 70% since mid-November, when Tesla"s debut in the S&P 500 was announced here, and have soared about 700% so far in 2020. Tesla jumped 6% on Friday in frantic trading ahead of its S&P 500 entry. GRAPHIC: Tesla joins the S&P 500 - Tesla’s addition to the S&P 500 led index-tracking funds to buy $90 billion of shares by the end of Friday so their portfolios reflected the index, according to S&P Dow Jones Indices’ analyst Howard Silverblatt. The change was effective prior to the open of trading on Monday, and Tesla is replacing Apartment Investment and Management Co. California-based Tesla’s rally has put its market value at about $630 billion, making it the sixth most valuable publicly listed U.S. company, though many investors view it as wildly overvalued. Some investors have speculated Tesla’s stock might sell off as demand subsides following its addition to the S&P 500. “The news is done now ... the question now is, what next?,” Keith Temperton, a sales trader at Forte Securities. Silverblatt said for every $11.11 Tesla moves, the S&P 500 changes 1 point, while the S&P’s 2021 price/earnings ratio rises to 22.6 from 22.3 following Tesla’s addition. The dividend yield for the S&P 500 after Tesla’s addition falls to 1.53% from 1.56%, he said. Tesla is by far the most traded stock by value on Wall Street, with $18 billion worth of its shares exchanged on average in each session over the past 12 months, easily beating Apple, in second place with average daily trades of $14 billion, according to Refinitiv. About a fifth of Tesla’s shares are closely held by Musk, the chief executive, and other insiders.