Pfizer and BioNTech have halted a U.S. study of their updated COVID-19 vaccine in adults ages 50-64 due to slow enrollment, a Pfizer spokesperson confirmed to Becker’s.
“We can confirm our communication to the FDA about the status of the Pfizer-BioNTech COVID-19 vaccine postmarketing activities, including our intent to stop study C4591081,” the spokesperson said. “Notably, this study is not ending as a result of any safety or benefit-risk concerns.”
The spokesperson added that the companies “intend to stop the study due to slow enrollment and therefore the inability to generate relevant postmarketing data.”
The study was designed as a postmarketing effort to evaluate vaccine performance in healthy adults ages 50-64. Recruitment challenges come as COVID-19 vaccination rates remain low, particularly among children. As of March 21, 9.3% of children had received the 2025-26 COVID-19 vaccine, according to CDC data.
The pause comes as the CDC continues to monitor emerging variants, including BA.3.2, known as the “cicada” variant, which has been identified in 23 countries. While U.S. prevalence remains low at 0.19%, early data suggests the variant may evade antibodies more effectively than other circulating strains.
The study also follows stricter FDA requirements for COVID-19 vaccines, including calls for large, placebo-controlled trials in this age group to support future recommendations.
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