In one case, a 67-year-old man infected a 6-month-old after a half-hour near the child.

The man was 12 days past his first positive Covid-19 test. He had taken a five-day course of Paxlovid and was feeling better. He didn’t have any symptoms when he saw the baby, who was his grandson, but about eight hours later, he started to feel ill again.

The baby tested positive about three days later, as did both of his parents. Neither the baby nor its parents had any other close contacts before they got sick.

“It indicates that you can transmit during rebound even before you develop symptoms,” Charness said. “And you know, we studied a small number of people. It’s certainly conceivable that there are other people out there who don’t have symptoms and still have a viral rebound.”