Pfizer pill questions after cases of Covid relapse
New questions are emerging about the performance of Pfizer’s powerful Covid-19 pill, including why a small number of patients appear to relapse after taking the drug.
Paxlovid has become the go-to option against Covid-19 because of its convenience and impressive results in preventing severe disease.
Doctors in New Zealand have been able to prescribe the drug since April, but the criteria for the pill are aimed at people more vulnerable to severe illness from Covid-19, such as those who are immunocompromised or who have chronic respiratory conditions or diabetes.
Why do some patients seem to relapse?
Doctors have started reporting rare cases of patients whose symptoms return several days after completing Paxlovid’s five-day regimen of pills.
That’s prompted questions about whether those patients are still contagious and should receive a second course of Paxlovid.
The US Food and Drug Administration has advised against a second round because there’s little risk of severe disease or hospitalisation among patients who relapse.
Dr Michael Charness, chief of staff at the VA Boston Healthcare System, reported last month on a 71-year-old vaccinated patient who saw his symptoms subside but then return, along with a spike in virus levels nine days into his illness.
Charness says Paxlovid remains a highly effective drug, but he wonders if it might be less potent against the Omicron variant. The treatment was approved based on its performance against the Delta variant.
Could some people just be susceptible to a relapse? Both the FDA and Pfizer point out that 1 per cent to 2 per cent of people in Pfizer’s original study saw their virus levels rebound after 10 days. The rate was about the same among people taking the drug or placebo, “so it is unclear at this point that this is related to drug treatment”, the FDA said.
Some experts point to another possibility: The Paxlovid dose isn’t strong enough to fully suppress the virus.
Andy Pekosz of Johns Hopkins University worries higher doses could spur drug resistant mutations.
“We should really make sure we’re dosing Paxlovid appropriately because I would hate to lose it right now,” said Pekosz, a virologist.
How well does Paxlovid work in vaccinated people? Pfizer tested Paxlovid in the highest-risk patients: unvaccinated adults with no prior Covid-19 infection and other health problems, such as heart disease and diabetes. The drug reduced their risk of hospitalisation and death from 7 per cent to just 1 per cent.
But that doesn’t reflect the vast majority of Americans today, where 89 per cent of adults have had at least one shot. And roughly 60 per cent of Americans have been infected at some point.
There’s no clear answer yet for vaccinated Americans, who already have a hospitalisation rate far below 1 per cent. That may come from a large, ongoing Pfizer study that includes high-risk vaccinated people. No results have been published; the study is expected to wrap up later this year.
Can Paxlovid be used to help prevent Covid-19 infection? Pfizer recently reported that proactively giving Paxlovid to family members of people infected with Covid-19 didn’t significantly reduce their chances of catching it.
But that’s not the end of the story: Pfizer is studying several other potential benefits of early use, including whether Paxlovid reduces the length and severity of Covid-19 among households.
“It’s a high bar to protect against infection but I’d love to see data on how Paxlovid did against severe disease because it may be more effective there,” said Pekosz.