Pfizer and BioNTech halted a large U.S. clinical trial targeting healthy adults aged 50 to 64 after enrollment shortfalls made it impossible to generate the post-marketing data the FDA required.
Pfizer and BioNTech halted a large U.S. clinical trial of their updated Covid-19 vaccine in healthy adults aged 50 to 64.
Both companies say enrollment shortfalls made it impossible to generate the data required by FDA to potentially expand the vaccine's recommended use to that age group.
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According to a report from Reuters, in a letter to trial investigators dated March 30, Pfizer said it would stop surveillance for Covid illness among all study participants after April 3, following a enrollment closure on March 6.1
The trial had aimed to enroll roughly 25,000 to 30,000 healthy adults aged 50 to 64, a population the FDA specifically required to be studied in a large, placebo-controlled trial before the vaccine could be included in recommendations for that group.
The companies were clear the decision was not driven by safety concerns. "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," Pfizer and BioNTech said.
Enrollment proved exceptionally difficult because participants were required to have no chronic conditions such as hypertension or diabetes, a stringent threshold that one contract research organization executive said caused more than 80% of willing participants to fail pre-screening.1
"This is a very difficult population to recruit," the executive said, speaking on condition of anonymity.
Currently, no Covid-19 vaccine has full FDA approval for healthy adults aged 50 to 64.1 Pfizer, BioNTech's, and Moderna's approved vaccines cover adults 65 and older, as well as younger people at higher risk of severe Covid-19, but without the trial data, approval for the 50 to 64 healthy adult group is now in doubt.1
The halt comes ahead of an expected May meeting of the FDA's Vaccines and Related Biological Products Advisory Committee, which typically uses study data to guide strain selection for autumn Covid vaccines.
Dr. Jesse Goodman, a former FDA chief scientist, said the absence of data likely means no presentation at the advisory committee meeting and potentially no specific approval for the age group.1
The companies did not comment on alternative plans for pursuing authorization in that population.
The trial's collapse reflects a confluence of pressures hitting Covid vaccine makers simultaneously. U.S. demand for Covid boosters has fallen sharply, with only about 18% of Americans receiving an updated shot during the 2025-26 season despite the virus continuing to cause an estimated 8,000 to 12,000 deaths annually among adults aged 50 to 64 alone.
The Trump administration has also
Moderna is running a similar U.S. trial targeting 30,000 volunteers, also facing recruitment challenges at multiple sites, though that study remains ongoing with an expected completion date of June 2027.1
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