Viral load ‘rebound’ has been observed in several patients treated with Paxlovid antiviral. The problem was reported by Pfizer in the dossier for the request for emergency authorization of the drug
US President Joe Biden, 79, returned positive at Covid about ten days after he recovered from the first infection, in which he had had mild symptoms. His doctor, Kevin O’Connor, explained that it is a “rebound” (rebound) of the positivity also observed in other patients treated with Paxlovid, the antiviral administered to the US president on the occasion of the first infection.
The clinical study
The drug, also authorized in Italy for adults with Covid who do not require oxygen therapy but they are at risk of serious illness, it must be administered as soon as possible and in any case within 5 days of the onset of symptoms. The therapy lasts 5 days and is almost 90% effective in preventing hospitalization for Covid. However, in recent months it has been seen that a relapse with return of positivity (in some cases with high viral load) can occur after treatment, just as happened to Biden. The problem was reported by Pfizer itself, which manufactures the drug, in the documentation attached to the application for authorization for emergency use (obtained in Europe on January 22). In the placebo-controlled clinical trial – which included 2,246 participants – “several subjects appeared to relapse after 10-14 days” from the start of treatment. Relapse affected approximately 2% of participants treated with the antiviral (compared with approximately 1.5% in those who received placebo).
But why can there be the rebound? The good news is that it does not appear to be drug “resistance” developed by the virus. One hypothesis is that early suppression of the pathogen could compromise the immune system’s response to infection; It is also possible that some people do not absorb the drug as effectively as others; last hypothesis, the body could harbor the coronavirus in places that Paxlovid would have difficulty accessing and after stopping treatment the small reserves of the virus could cause relapse. Experts wonder why in the Pfizer clinical study the relapses were few (2%) and almost at the same level as those observed in the placebo group (1.5%), while now in clinical practice they seem much more frequent. One answer might relate to Pfizer’s study population in the study, different from the patients for whom Paxlovid is typically prescribed. These were unvaccinated subjects, with high risk factors and infected with the Delta variant. Today, patients are generally vaccinated, have a lower risk and are infected with Omicron.
Can it be transmitted?
Another important aspect concerns the possibility of transmitting the virus even during the rebound and in the complete absence of symptoms. Experts are quite convinced that this is the case. «People who experience a rebound they can infect others, even if they are outside what is considered the usual window of transmission “he explained to the Cnn Michael Charness of the Veterans Administration Medical Center in Boston, who together with colleagues and a group of researchers from Columbia University examined several cases of relapse after treatment with Paxlovid. In at least two cases there was a transmission of the infection during the rebound. “People take Paxlovid and we know it works very well by blocking viral replication,” Charness said. But then in some – it is not known how many, because not enough people have been studied – the levels of the virus begin to rise ».
More studies are needed
According to Charness, the fact that the infection can recur after treatment raises some questions. For example, the rebound it would be equally frequent in people who start therapy later, on the fourth fifth day after the first symptoms, so that the immune system has more time to recognize the virus? Would a longer course of treatment – perhaps taking the drug for 6 or 7 days instead of 5 – reduce the risk of the virus returning? “Nobody knows, we need studies” concludes the expert.
In the United States, prescriptions for Paxlovid are on the rise, from 27,000 to approximately 182,000 per week, according to a White House report. In May, the Centers for Disease Control and Prevention (CDC) published guidelines for post-treatment relapses. Those who test positive after the end of antiviral therapy, it says, must go back to isolation for 5 days and wear the mask for 10 days. In many cases, relapses are asymptomatic, but there have also been patients with symptoms equal to or worse than those of the original disease.
August 1, 2022 (change August 1, 2022 | 19:15)
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Biden positive again within days of recovery: what could be the cause?
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