As reports of ‘Paxlovid rebound’ increase, Covid researchers scramble for answers

As he was treating some of the nation’s first coronavirus patients, Andre Kalil noticed something unusual about the new virus: Patients didn’t always progress linearly. They’d get better, then worse. Then sometimes better again.

Initially, most researchers figured these undulating symptoms were collateral damage, as a riled-up immune system kept firing long after most of the virus was gone. Sometimes, though, Kalil could swab the lungs of a patient in the ICU and find virus still replicating weeks after they were admitted. Often, the amount of virus bounced up and down by the day.