New York, Nov 14 : While US pharma Pfizer's antiviral Paxlovid remains a lifesaving drug, a new study showed that one in five individuals experienced Covid rebound. The study by Harvard Medical School (HMS) researchers at Massachusetts General Hospital and Brigham and Women's Hospital found that about 20 per cent people taking nirmatrelvir-ritonavir therapy, commonly known as Paxlovid, to treat severe symptoms of Covid-19 had a positive test result and shed live potentially contagious virus following an initial recovery and negative test -- a phenomenon known as virologic rebound. By contrast, only two per cent people not taking Paxlovid experienced rebound. Individuals with rebound also had prolonged viral shedding, for an average of 14 days compared with fewer than five days in those who did not experience rebound, indicating they may remain contagious for a longer period. Reassuringly, there was no evidence that the virus is developing resistance to the medication among patients with rebound. The results, published in the 'Annals of Internal Medicine', indicate that viral rebound may occur more often than previously believed and raise questions about the risk of viral transmission by those who do experience a return of the virus. 'We conducted this study to address lingering questions about Paxlovid and virologic rebound in Covid-19 treatment,' said Mark Siedner, Associate Professor of Medicine at HMS and an infectious disease clinician and researcher at Mass General. 'We found that the virologic rebound phenomenon was much more common than expected -- in over 20 per cent of people taking Paxlovid ' and that individuals shed live virus when experiencing a rebound, which means they may be contagious after initial recovery,' he added. Paxlovid is an oral antiviral medication used to treat Covid-19. Previous studies demonstrate the medication's effectiveness in reducing hospitalisation and death in cases of severe Covid. The new findings, based on 142 individuals, should not discourage clinicians from prescribing the medication, the researchers noted, but they should prompt them to counsel patients who take the medication about the risk for viral rebound and spreading the virus to others. "Paxlovid remains a lifesaving drug I prescribe to high-risk patients," said Jonathan Li, Associate Professor of medicine at HMS and an infectious disease physician at Brigham and Women's. 'This study, while informative, does not change the fact that this drug is very effective at preventing hospitalisations and death. Instead, it offers valuable insights to Paxlovid patients, helping them understand what to expect and how long they might be contagious,' said Li. The researchers caution that the study relied on observation and was not a randomised controlled trial, so the scientists cannot be certain that the increased rebound rate seen in people taking Paxlovid was solely due to the use of the drug. /IANS