Editor’s Note: This story originally published on May 31, 2022.

CNN  — 

People who have a Covid-19 rebound after treatment with the antiviral drug Paxlovid can be contagious and may not know it because they might not have symptoms, researchers warn.

“People who experience rebound are at risk of transmitting to other people, even though they’re outside what people accept as the usual window for being able to transmit,” said Dr. Michael Charness of the Veterans Administration Medical Center in Boston.

Charness and his colleagues recently collaborated with a team of researchers at Columbia University to look into cases of Covid-19 that return after Paxlovid treatment. He said they’ve found at least two instances in which people have transmitted to others when their infection recurs.

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.”

In another instance, a 63-year-old man infected two family members during three days of relapse after Paxlovid.

Take precautions after Paxlovid

Based on this research, the US Centers for Disease Control and Prevention issued new guidance last week for people experiencing Covid-19 rebound after Paxlovid.