But as the treatment spreads, so too does confusion over its effectiveness and side effects. Patients have complained of a bitter, metallic taste, or one like grapefruit juice mixed with soap, the whole time they were on the drug. More concerning, some have reported experiencing a second round of symptoms, and going back to testing positive, when the pills were done, a phenomenon that’s become known as “Paxlovid rebound.” Meanwhile, Pfizer has never published any final data on the use of the drug by vaccinated patients, leaving medical professionals with little information about how the drug works for people who have received their shots—which is to say, most of the adult population in the U.S. “We’re all riding on hope at this point,” Reshma Ramachandran, a family-medicine doctor at Yale, told me.

An individual patient would never know if Paxlovid worked for them, because you could never say how sick you would have gotten if you hadn’t taken the pills. If the drug doesn’t really do that much for vaccinated people—if it fails to have meaningful effects on their risk of severe disease, and doesn’t help resolve their symptoms—then giving it out widely could be a waste of the dwindling resources the United States has committed to fight the pandemic, not to mention physicians’ time and patients’ sense of taste. And because people who have just recovered from COVID might reasonably believe they’re in the clear, and mingle with abandon, surprise cases of Paxlovid rebound could end up causing more transmission. “We continue to monitor data from our ongoing clinical studies and post-authorization safety surveillance,” a Pfizer spokesperson told me in an email, noting that cases of viral rebound “are being reported at a rate consistent with observations” from the company’s published clinical trial.