Pfizer and BioNTech have halted a major U.S. clinical trial of their updated COVID-19 vaccine in adults aged 50 to 64, citing insufficient participant enrollment to produce reliable data.

In a March 30 letter to investigators, seen by Reuters, Pfizer said it would stop monitoring participants for COVID-19 illness after April 3, effectively ending the study’s active surveillance phase.

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.

Two sources from companies that manage 18 of the 208 trial sites said Pfizer told them in writing in early March to stop recruitment.

The trial aimed to test the efficacy, immune response and safety of an updated COVID vaccine from Pfizer and BioNTech in healthy adults aged 50 to 64. The FDA required participants to have no chronic conditions such as hypertension or diabetes.

"This is a very difficult population to recruit," said one executive at a contract research organization involved in the trial, who spoke on the condition of anonymity.

"Even when patients are willing to participate in COVID studies, more than 80% fail at pre-screening because they don't meet the health criteria. It's been a ⁠real challenge to enroll enough patients, particularly given the scale of these trials," the executive said.

The Pfizer-BioNTech study halt comes ahead of an expected May meeting of the FDA's Vaccines and Related Biological Products Advisory Committee, which would use study data to guide decisions on strain selection for COVID vaccines this autumn.

"Without the data, there ​probably just won't be a presentation," said Dr. Jesse Goodman, a former FDA chief scientist. "And without the presentation, there may not be a specific approval ​for this age group."

Currently, no COVID-19 vaccines have full FDA approval for healthy adults aged 50 to 64. Pfizer and BioNTech's Comirnaty and Moderna's Spikevax and next generation mNEXSPIKE are approved for adults aged 65 ⁠and older, as ‌well as for ‌younger people who are at higher risk of severe COVID-19.

Moderna is running a similar U.S. study ⁠with an aim to enroll 30,000 volunteers, and is also facing recruitment challenges, sources ‌at four sites said. That study is expected to be completed in June 2027. Moderna did not immediately respond to a Reuters request for comment.

COVID vaccines remain significant products ​for their makers, though demand has fallen sharply from ⁠pandemic peaks.

Only about 18% of Americans received a COVID booster during the 2025-26 season, according to ⁠data from the U.S. Centers for Disease Control and Prevention, even as the virus continues to cause tens of thousands of deaths each ⁠year, including an estimated 8,000 to ​12,000 among adults aged 50 to 64.

Updated COVID vaccines for approved groups are typically cleared based on immune response data showing how well a new formulation generated antibodies against circulating variants, similar to the agency's approach for annual flu shots.