Pfizer, BioNTech End mRNA COVID-19 Biologic Trial Early

After failing to recruit enough eligible volunteers to generate enough data in the U.S. to meet U.S. Food and Drug Administration (FDA) post-marketing evidence requirements, Pfizer, Inc. and BioNTech SE have halted a large clinical trial of their updated mRNA COVID-19 biologic.  The study targeted healthy adults 50 to 64 years of age but fell significantly short of its clinical trial participant enrollment goals, in part because they couldn’t find enough healthy volunteers without chronic poor health conditions. The companies discontinued enrollment in the study on Mar. 6, 2026 and said they would stop monitoring participants for signs of COVID illness after Apr. 3, 2026.¹

Pfizer and BioNTech said that product safety concerns did not drive the decision and emphasized that low enrollment prevented them from generating sufficient data for regulatory evaluation. They stated:

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.²

Strict Eligibility Rules and Declining Demand Hindered Enrollment

The study’s design played a central role in the recruitment challenges as it focused on “healthy” adults aged 50 to 64 and excluded individuals with common chronic conditions, such as hypertension, diabetes, and obesity. Since these poor health conditions are widespread, more than 80 percent of potential participants failed the screening process for the study. This narrow eligibility pool significantly limited enrollment among individuals willing to participate in the trial.³

Pfizer and BioNTech had planned to enroll about 25,000 to 30,000 healthy adults aged 50 to 64, as the FDA required a large, placebo-controlled trial in this age group before including the mRNA COVID shot in its federal vaccine recommendations.⁴ Currently, the FDA has not granted full licensing approval for any COVID shots that target healthy adults aged 50 to 64. FDA regulators have only approved Comirnaty, manufactured by Pfizer and BioNTech, as well as Moderna’s Spikevax and its next-generation version, mNEXSPIKE, for adults 65 years of age and older and for younger individuals who face a higher risk of having severe complications when infected with the SARS-CoV-2 virus.⁵

At the same time, there is declining demand for COVID shots in the U.S., with uptake falling to around 18 percent during 2025–2026.⁶

The study was part of ongoing efforts to gather additional data for COVID shots used by lower-risk populations under FDA guidance. The FDA requires strong clinical evidence to support vaccine use in populations at lower risk of severe disease in an effort to ensure that potential benefits outweigh potential risks. These evidence requirements reflect standard regulatory practice and become more important as SARS-CoV-2 has become more endemic in the U.S. population.⁷

The halt to the study has taken place shortly before the FDA’s Vaccines and Related Biological Products Advisory Committee (VRBPAC) is scheduled to meet in May 2026 , when members typically evaluate scientific data to determine which SARS-CoV-2 vaccine strains will be included in updated COVID shots for the fall season. Jesse Goodman, MD, MPH, a former FDA chief scientist, said the missing data from the study that was ended early will likely prevent a presentation on COVID shot updates at the meeting and may delay or limit approval for the healthy adult age group.⁸

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