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The FDA’s stricter trial requirements and low vaccine use in the United States are hindering the study’s progress
Pfizer and BioNTech cite slow recruitment, not safety issues, as the reason for halting the trial
Moderna’s study is facing similar challenges, according to sources
(Adding context, an attempt to contact the FDA and companies in paragraph 10; update on actions in paragraph 8)
by Bhanvi Satija
Vaccine manufacturers Pfizer and BioNTech have halted a large U.S. trial of their updated COVID-19 vaccine in healthy adults aged 50 to 64, stating that the trial enrollment numbers were too low to generate necessary data.
In a letter dated March 30 addressed to trial investigators, Pfizer declared that they would stop monitoring COVID-related disease signs for all study participants after April 3.
Recruitment closed on March 6, following a review of current epidemiological trends.
This decision comes as COVID vaccine manufacturers face resistance from U.S. authorities and low vaccine demand in the United States.
Last year, the Food and Drug Administration (FDA) tightened its requirements for COVID vaccine use, including requesting large-scale placebo-controlled trials in the 50-64 age group for the vaccine to be included in recommendations.
Pfizer and BioNTech informed Reuters that they had notified the FDA of their intention to halt the COVID-19 vaccine study, citing difficulty in recruiting a sufficient number of participants. The goal was to recruit between 25,000 and 30,000 participants.
“This study is not being halted due to safety issues or risk-benefit concerns. We intend to stop the study due to slow recruitment, thus the inability to generate relevant post-marketing data,” the companies stated.
Pfizer and Moderna, a competitor COVID vaccine manufacturer, saw their stocks rise by over 1%, while BioNTech’s U.S.-listed shares increased by about 2%.
FDA vaccine chief Vinay Prasad, who advocated for placebo-controlled trials on healthy adults and children, is leaving the agency this month. Experts have warned that requiring new large-scale trials could delay or limit the availability of updated vaccines for low-risk groups.
The path to approval for this group was not clear. The FDA was not immediately available for comment. The companies did not comment on vaccine marketing plans for this age group.
The Pfizer-BioNTech study halt comes ahead of a planned May meeting of the FDA advisory committee on vaccines and biologics, which will use study data to guide decisions on selecting strains for COVID vaccines this fall.
“Without the data, there will likely be no presentation,” said Dr. Jesse Goodman, former FDA chief scientist. “And without that presentation, there may not be specific approval for this age group.”
A CHALLENGING POPULATION TO RECRUIT
Two sources from companies managing 18 of the 208 trial sites stated that Pfizer had requested, in writing in early March, to stop recruitment.
The trial aimed to test the efficacy, immune response, and safety of an updated COVID vaccine by Pfizer and BioNTech in healthy adults aged 50 to 64. The FDA required participants without chronic illnesses such as hypertension or diabetes.
“This is a very challenging population to recruit,” said a research organization executive participating in the trial, speaking anonymously.
<p"Even when patients agree to participate in COVID studies, over 80% of them fail pre-screening because they do not meet health criteria. Recruiting a sufficient number of patients has been a real challenge, especially given the scale of these trials," the executive said.
MODERNA TRIAL CONTINUES; COVID VACCINE DEMAND DROPS
Currently, no COVID-19 vaccine has been approved by the FDA for healthy adults aged 50 to 64. Pfizer and BioNTech’s Comirnaty and Moderna’s new-generation Spikevax and mNEXSPIKE are approved for those aged 65 and older, as well as younger individuals at higher risk of severe COVID-19 infection.
Moderna is conducting a similar study in the U.S. to recruit 30,000 volunteers, but it is also facing recruitment challenges, sources at four sites indicated. This study is expected to conclude in June 2027. Moderna did not immediately respond to a request for comment from Reuters.
COVID vaccines remain important products for their manufacturers, although demand has decreased significantly compared to pandemic peaks.
Only 18% of Americans received a COVID booster during the 2025-26 season, according to data from the Centers for Disease Control and Prevention, while the virus continues to cause tens of thousands of deaths each year, including 8,000 to 12,000 in adults aged 50 to 64, according to estimates.
Updated COVID vaccines for approved groups are typically authorized based on immune response data showing how a new formulation generated antibodies against circulating variants, similar to the agency’s approach for annual flu vaccines.
