
A more contagious variant of the coronavirus, known as delta, is spreading in the United States and around the world, causing a surge of cases in some countries and prompting several nations to introduce new lockdowns.
The delta variant, which was first identified in India, now accounts for 25 percent of new Covid-19 cases in the U.S., and is on track to become the dominant version of the virus circulating in the country, according to the Centers for Disease Control and Prevention.