Pfizer and BioNTech said they will pause enrollment in a large U.S. post-marketing COVID-19 vaccine study due to slow recruitment. The trial was designed to enroll healthy adults aged 50 to 64 and was intended to generate relevant post-marketing data under FDA-imposed requirements. Company statements cited recruitment shortfalls that prevent collection of the planned dataset, rather than any new safety or benefit-risk concerns. Pfizer said it would suspend the study after March enrollment checkpoints and ongoing surveillance limitations. The pause reflects broader challenges facing COVID vaccine trial operations, including changing epidemiology, stricter regulatory design expectations, and reduced availability of placebo-controlled structures for the appropriate population. For developers and CROs, the decision highlights how enrollment constraints can derail post-authorization evidence generation, especially in large, age-restricted cohorts when background incidence shifts.