The U.S. Centers for Disease Control and Prevention (CDC) issued an emergency advisory this week, warning that some American who take Pfizer's
Paxlovid was authorized for emergency use by the U.S. Food and Drug Administration (FDA) back in December for people diagnosed with COVID aged 12 years and older who have mild or moderate symptoms but are at risk of severe infection.
The drug course include two Paxlovid pills and one tablet of a common HIV drug developed by Pfizer (ritonavir) taken twice daily for five days. However, two to eight days after some patients complete the treatment and receive a negative test, they may test positive again and symptoms may reappear, the CDC warned.
The agency's warning backed a recent preprint case posted last month that sequenced virus samples from a 71-year-old patient who experienced COVID rebound after completing a full course of Paxolivd treatment, NBC reports. The study found no evidence that the patient developed tolerance to the drug, but instead suggests that symptoms reoccur before the immune system naturally neutralizes the virus, NBC reports.
Pfizer's clinical trial for the antiviral demonstrated that it lowered the risk of hospitalization or death by nearly 90% in high-risk patients when given within five days of symptom onset. The CDC said that COVID rebound does not mean that Paxlovid is not effective, but "may be part of the natural history: of infections for some patients, regardless of if they took Paxlovid or their vaccination status.
The CDC has not identified any cases of severe disease among patients whose symptoms reoccur after taking Paxlovid, and no patient required additional treatments. The CDC found on average COVID rebound symptoms return or patients re-test positive within three days of their symptoms resolve. The CDC added that cases of COVID rebound do not appear to be connected to re-infection.
The CDC recommends COVID rebound patients to restart their 5-day isolation period.
Pfizer said in its recent earnings call in April that Paxlovid prescriptions increased nearly 10-fold in the United States since February. The company said that Paxlovid currently dominates 90% of market share compared to Merck's
