COVID rebounds: Immune responses may be reignited by cleanup of viral scraps

Enlarge / A box of Paxlovid, the Pfizer antiviral drug. (credit: Getty | Europa Press News)

Pfizer's antiviral pill Paxlovid is among the most treasured tools for hammering COVID-19; it can knock back the relative risk of hospitalization and death by 89 percent in unvaccinated patients at high risk of severe disease. But, as use of the convenient drug has grown in the US, so have troubling reports of rebound cases—people who took the pill early in their infection, began feeling better, and even tested negative but then slid back into symptoms and tested positive again days later.

It's still unclear just how common the phenomenon is, but it certainly happens in some proportion of Paxlovid-treated patients. In May, the Centers for Disease Control and Prevention even issued a health alert over the rebound reports.

But, amid the rising awareness, it has also become clear that patients who have not been treated with Paxlovid can also rebound. In fact, in Pfizer's clinical trials of Paxlovid, researchers noted that about 1 percent to 2 percent of both treatment and placebo groups had rebounds.