A team at Imperial College London used cell phone data from more than 10 million people to calculate that 65 of 100 infections still originated from those aged 20 to 49 in the US.

The found that people in that age bracket accounted for about 72 percent of the cases after schools reopened in October. Less than 5 percent came from children, and less than 10 percent from teens.

Adults aged 35 to 49 accounted for 41 percent of new cases through mid-August, compared to 35 percent for adults aged 20 to 34, according to the peer-reviewed study published in Science.