Pfizer, BioNTech Halt COVID-19 Vaccine Trial Over Enrollment

Pfizer and BioNTech announced on March 30, 2026, that they have halted a large-scale U.S. clinical trial for their updated COVID-19 vaccine targeting healthy adults aged 50 to 64, after failing to enroll enough participants to produce meaningful data. The trial, which aimed to assess efficacy, immune response, and safety, sought between 25,000 and 30,000 volunteers but closed enrollment on March 6 following a review of epidemiological trends. Surveillance for COVID-19 illness among participants will end after April 3.

The companies informed the U.S. Food and Drug Administration of their decision, emphasizing that the halt stems solely from recruitment difficulties rather than any safety or benefit-risk issues. "This study is not ending as a result of any safety or benefit-risk concerns," Pfizer and BioNTech stated in a letter to trial investigators. "We intend to stop the study due to slow enrollment and therefore the inability to generate relevant postmarketing data."

Recruitment proved challenging for the study, which required participants to be healthy individuals without chronic conditions such as hypertension or diabetes. An executive at a contract research organization...