Pfizer and BioNTech announced on Apr. 2 that they have stopped enrollment in a major U.S.-based trial of their updated COVID-19 vaccine, citing slow recruitment as the main reason.
The halted study was designed to enroll between 25,000 and 30,000 healthy adults aged 50 to 64. Pfizer reportedly closed enrollment on March 6 after reviewing current epidemiological trends, according to Reuters. The company also told trial investigators on March 30 that it would end surveillance for signs of COVID-19 illness in the study after April 3.
Both companies said they are stopping the study because slow enrollment is preventing them from generating relevant post-marketing data for the vaccine. An unnamed executive at a contract research organization involved in the trial said, “This is a very difficult population to recruit.” The executive added, “Even when patients are willing to participate in COVID studies, more than 80% fail at prescreening because they don’t meet the health criteria. It’s been a real challenge to enroll enough patients, particularly given the scale of these trials.”
The decision comes after new requirements from the Food and Drug Administration last year. In May, the FDA announced it would require data from placebo-controlled trials before supporting use of COVID-19 vaccines for people aged 50 to 64. Currently, neither Pfizer and BioNTech’s Comirnaty nor Moderna’s Spikevax and next generation mNEXSPIKE has full approval for this age group.
Moderna is also conducting a similar vaccine trial among people aged 50 to 64 but faces similar challenges with participant recruitment at several sites involved in its study. While Moderna has not commented directly on its own enrollment rate yet, commercial factors may be affecting interest levels among potential participants.
Looking ahead, Moderna's chief financial officer James Mock projected that U.S. sales could fall from $1.2 billion this year to about $1 billion in 2026 amid ongoing political and regulatory pressure.
