Pfizer and BioNTech are stopping a large U.S. phase 3 study of an updated COVID-19 vaccine in healthy adults ages 50 to 64 after slow recruitment left the trial unable to generate the data needed for the regulatory package, Reuters reported. The companies said the move was not based on safety or benefit-risk concerns and that they had notified the FDA.

Pfizer and BioNTech are halting a large U.S. phase 3 study of their updated COVID-19 vaccine in healthy adults ages 50 to 64 after recruitment fell short of what was needed to support the regulatory data package, Reuters reported on Wednesday.

The companies said the decision was not driven by safety or benefit-risk concerns. They also said they had informed the U.S. Food and Drug Administration.

According to Reuters, Pfizer told trial investigators in a March 30 letter that surveillance for COVID illness in the study would stop after April 3. The report said the study had struggled to enroll enough participants to deliver the data the companies wanted.

ClinicalTrials.gov lists the related study, C4591081 / NCT07300839, as a placebo-controlled phase 3 trial in healthy adults 50 through 64 years old with an estimated 25,500 participants. The registry says the trial began on Dec. 10, 2025 and had an estimated primary completion date of July 31, 2026.

The setback comes as vaccine makers continue to navigate demand and operational challenges around updated COVID shots, even as the products remain part of the broader public health response.

Revision note

Initial automated publication.