The US FDA has launched an investigation after reports of COVID relapses in patients within 5 to 9 days following the use of Pfizer’s pill known as nirmatrelvir.
The U.S. Food and Drug Administration (FDA) is looking into reports of relapses among people who used Pfizer’s COVID-19 pill.
An FDA spokesman said in an email that the agency is “evaluating the reports of viral load rebound after completing paxlovid treatment and will share recommendations if appropriate.”
A 71-year-old man who took the pill, also known as nirmatrelvir, witnessed a “rapid and progressive reduction” in the viral load of SARS-CoV-2, the virus that causes COVID-19, according to a recent preprint case report from Veterans Affairs researchers.
However, four days after finishing the treatment, they reported a “surprising rebound of viral load and symptoms.”
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According to the researchers, the report “highlights the potential for recurrent, symptomatic SARS-CoV-2 replication after successful early treatment” with the pill.
Others have reported experiencing new symptoms after taking paxlovid.
In an ongoing phase 2/3 trial undertaken by Pfizer, several participants “appeared to have a rebound” in viral load five to nine days after finishing their treatment cycles, according to the FDA’s assessment (pdf given below) of data on paxlovid, which the agency cleared on an emergency basis in 2021.
Additional analyses of the paxlovid trial data were conducted in light of the new reports, which revealed that 1 to 2% of patients had one or more positive COVID-19 tests after testing negative, or an increase in the amount of viral load, after completing the treatment, according to Dr. John Farley of the FDA in an interview published on May 4.
“This finding was observed in patients treated with the drug as well as patients who received placebo, so it is unclear at this point that this is related to drug treatment,” he said, adding that, at this time, the reports “do not change the conclusions from the paxlovid clinical trial which demonstrated a marked reduction in hospitalization and death.”
Pfizer must later disclose information about “prolonged virologic shedding or rebound in clinical trials” as part of the authorization agreement, according to the FDA.
A request for comment from Pfizer was not answered.
According to Bloomberg, the rate of rebound in the company’s trial was not greater among paxlovid users than among placebo users.
The company concluded that “this suggests the observed increase in viral load is unlikely to be related to paxlovid.”
The National Institute of Allergy and Infectious Diseases’ deputy director for clinical research, Dr. Clifford Lane, told the publication that the agency will investigate the matter, calling it “a priority.”
Lane and the agency did not respond to requests for comment.
Paxlovid has been approved by the FDA for the treatment of mild to moderate COVID-19 in people aged 12 and up. A person must test positive for COVID-19 and be considered at high risk of developing serious disease in order to receive the pill.
The FDA assessment is given below: