Paxlovid is an oral two-drug combination regimen that treats COVID-19. One drug is nirmatrelvir, an antiviral, while the second, ritonavir, inhibits the metabolism of the nirmatrelvir, maintaining therapeutic levels.
Although not perfect, Paxlovid is a breakthrough, with efficacy over 90% in treating mild-to-moderate COVID-19 and preventing hospitalization and death.
However, physicians are seeing an unusual phenomenon in a small number of patients treated with Paxlovid – an improvement of symptoms while taking the drug, only to have them return after completion of the five-day course. It has been dubbed “Paxlovid Rebound.” There are not much data on how frequently this occurs, but Pfizer’s application to the FDA for Emergency Use does state that “several subjects appeared to have a rebound in SARS-CoV-2 RNA levels around Day 10 or Day 14.”
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Whatever the frequency, there are several possible mechanisms to explain such a phenomenon… While a five-day course of Paxlovid is sufficient to keep about 90% of COVID victims out of the hospital and reduce their viral load 10-fold, there may be a subpopulation of people who either didn’t respond as well as others to Paxlovid or, perhaps, had a higher viral load to begin with.
