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

Upon first inspection, the mutations in the highly contagious delta covid variant don’t look that worrisome. 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. But those expectations were wrong. 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. Delta is more dangerous in many ways. It has an incubation period of four days, rather than six, making people contagious sooner. When the pandemic began, people spread the original coronavirus to an average of two or three people. Today, people infected with delta infect six people, on average. As of this week, the delta variant had...