Vaccine makers Pfizer and BioNTech halted a large U.S. trial of their updated COVID-19 vaccine in healthy adults aged 50 to 64, saying enrollment in the trials had been too low to generate the needed data.
In a letter to trial investigators dated March 30, seen by Reuters and previously unreported, Pfizer said it would stop surveillance for signs of COVID illness of all participants in the study after April 3.
Enrollment was closed on March 6, following a review of current epidemiological trends, it said.
The move comes as COVID vaccine makers grapple with pushback from the U.S. administration and weak U.S. demand for the shots.
The U.S. Food and Drug Administration toughened requirements for COVID vaccine use last year, including asking for large, placebo-controlled trials in the 50-64 age group for it to be included in recommendations.
Pfizer and BioNTech told Reuters they had informed the FDA about their intent to halt the COVID-19 vaccine study, citing challenges getting enough participants. The target enrollment was roughly 25,000 to 30,000 participants.
"This study is not ending as a result of any safety or benefit-risk concerns. We intend to stop the study due to slow enrollment and therefore the inability to generate relevant post-marketing data," the companies said.
Shares of Pfizer and U.S.-listed shares of BioNTech were up about half a percent, while rival COVID-vaccine maker Moderna was up 2%.
FDA vaccine chief Vinay Prasad, who had pushed for the placebo-controlled trials in healthy adults and children, is leaving the agency this month. At the meeting of the FDA's vaccine advisers last year, some experts warned that requiring large new trials could delay or limit the availability of updated shots for lower-risk groups.
The FDA was not immediately available for comment.
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