Unraveling the mysterious mutations that make delta the most transmissible Covid virus yet

For starters, delta has fewer genetic changes than earlier versions of the coronavirus. “When people saw that the epidemic in India was driven by delta, they did not suspect it would be so bad or overtake other variants,” said Trevor Bedford, an evolutionary biologist at the Fred Hutchinson Cancer Research Center. Delta has kept some of the most successful mutations found in earlier variants, but also contains new genetic changes that enable it to spread twice as fast.