Closeup of man getting ready to take a pill with water

The World Health Organization says it strongly recommends that people with mild or moderate COVID-19 illness at risk for hospitalization take the antiviral Paxlovid. The news comes as more instances of recurring symptoms are being reported after taking the drug.

Pfizer’s course of oral pills containing ritonavir-boosted nirmatrelvir should be considered for patients with nonsevere illness who are unvaccinated, elderly or immunocompromised, the WHO announced Friday. In these cases, WHO favors Paxlovid over remdesivir, Merck’s antiviral pill molnupiravir and COVID-19 monoclonal antibody treatments, the Agence France-Presse reported. 

In the United States, the federal COVID-19 Treatment Guidelines Panel also recommends that vulnerable patients with nonsevere illness be treated with Paxlovid. The drug should be initiated within five days of symptom onset, according to the Food and Drug Administration. 

The FDA authorized Paxlovid in December for emergency use during the pandemic. The authorization pleased clinicians. In clinical trials, the drug reduced the rate of hospitalization and death from COVID-19 by nearly 90%. And it was one of the first COVID-19 treatments that did not require intravenous infusion. Remedesivir, also a highly effective antiviral, requires intravenous infusion. Molnupiravir, meanwhile, is less effective than Paxlovid.

Symptoms rebound after treatment

But concerns have recently been raised by about a phenomenon that may follow use of Paxlovid. In a flurry of social media posts, patients and clinicians have reported an odd turn of events affecting some of those who are treated with the medication, according to the Boston Globe. Many describe taking a five-day course of the drug and testing negative for SAR-CoV-2 infection, only to experience a return of symptoms and sometimes a new positive result on a COVID-19 test.

Scientists are still trading theories about why this is happening, but some clinicians and scientists have expressed concern that these patients may still be infected and transmitting the disease.

“If you are testing positive on a rapid antigen test, then you have a decent amount of virus and likely an infectious amount of virus,” one infectious disease expert told the news outlet.