US stock index futures extended declines on Thursday after data indicated initial jobless claims unexpectedly increased for the previous week, adding to concerns that surging coronavirus cases will suffocate growth in the world’s biggest economy.At 8:33 a.m. ET, Dow e-minus dropped by 76 points, or 26%, S&P 500 e-minus dropped by 10 points, or 2.8%, and Nasdaq 100 e-minis dropped by 30.5 points, or 2.6%.Moments before the data, Dow e-minis dropped by 55 points, or 0.19%, S&P 500 e-minis dropped by 5.5 points, or 15%, and Nasdaq 100 e-minis dropped by 18.5 points, or 16%.