The Standard & Poor’s 500 dropped 2.3% on Friday, its worst one-day loss since February. Only five trading days earlier, the S&P 500 had closed at a record high. Friday’s abrupt reversal occurred after a handful of nations, including the United States, reinstated travel restrictions on visitors coming from southern Africa, a precaution taken to prevent the spread of a new variant of Covid-19. Whether the new variant is prevented..