In an indication of the growing resistance to vaccinations and amid a groundswell of pushback from both the US administration and weak public demand, Pfizer and BioNTech have stopped a large US trial of their updated Covid-19 vaccine in healthy adults aged 50 to 64, saying enrolment had been too low, reports Reuters.
Pfizer told investigators, in a letter dated 30 March, that it would stop surveillance for signs of Covid illness for all participants in the study after 3 April.
Enrolment was closed on 6 March, after a review of current epidemiological trends, it said.
The US Food and Drug Administration toughened requirements for Covid vaccine use last year, including asking for large, placebo-controlled trials in the 50-64 age group for it to be included in recommendations.
Pfizer and BioNTech said they had informed the FDA about their intent to halt the study, citing challenges in garnering enough participants to generate the data they needed. The target enrolment was roughly 25 000 to 30 000 candidates.
Experts have warned that requiring large new trials could delay or limit the availability of updated shots for lower-risk groups.
The Pfizer-BioNTech study has been stopped on the eve of an expected May meeting of the FDA’s Vaccines and Related Biological Products Advisory Committee, which would use study data to guide decisions on strain-selection for Covid vaccines this year.
Difficult population to recruit
Two sources from companies that manage 18 of the 208 trial sites said Pfizer wrote to them in early March to stop recruitment.
The trial, which aimed to test the efficacy, immune response and safety of an updated Covid jab, had a stipulation from the FDA that participants had no chronic conditions like hypertension or diabetes.
“This is a very difficult population to recruit,” said one executive at a contract research organisation involved in the trial.
“Even when patients are willing to participate in Covid studies, more than 80% fail at pre-screening because they don’t meet the health criteria. It’s been a real challenge to enrol enough patients, particularly given the scale of these trials.”
Moderna trial goes on
Currently, no Covid-19 vaccines have full FDA approval for healthy adults aged 50 to 64. Pfizer and BioNTech’s Comirnaty and Moderna’s Spikevax and next-generation mNEXSPIKE are approved for adults aged 65 and older, as well as for younger people who are at higher risk of severe Covid-19.
Moderna is running a similar US study with an aim to enrol 30 000 volunteers, and is also facing recruitment challenges, sources at four sites said. That study is expected to be completed in June 2027.
Moderna did not immediately respond to a Reuters request for comment.
Covid vaccines remain significant products for their makers, though demand has fallen sharply from pandemic peaks. Only about 18% of Americans received a booster shot during the 2025-26 season, according to data from the US Centres for Disease Control and Prevention, even as the virus continues to cause tens of thousands of deaths each year, including an estimated 8 000 to 12 000 among adults aged 50 to 64.
Updated Covid vaccines for approved groups are typically cleared based on immune response data showing how well a new formulation generated antibodies against circulating variants, similar to the agency’s approach for annual flu shots.
See more from MedicalBrief archives:
FDA global standing at risk with Covid vaccine deviations
Novavax trials in SA and UK confirm high efficacy against original and variants
US reverses Covid vaccine guidelines, calls for new scientific evidence