Carl Zimmer -- April 24, 2026
Regulator demands fuller cardiac and inflammatory data after several Phase 3 participants reported adverse reactions.
CAMBRIDGE, Mass. — Moderna has paused its Phase 3 trial of mRNA-1018, its experimental H5N1 avian influenza vaccine, after a safety signal emerged in the first weeks of dosing, the company confirmed Wednesday. The U.S. Food and Drug Administration has placed the study on partial clinical hold and requested additional data before enrollment can resume.
The trial, which began enrollment in early April, had dosed roughly 4,000 healthy adults across 38 sites in the United States and the United Kingdom. According to a statement from Moderna, an independent Data Safety Monitoring Board flagged a higher-than-expected rate of myocarditis and pericarditis cases among participants who received the higher 50-microgram dose, prompting the voluntary pause. The lower 25-microgram arm was not implicated in the safety signal, the company said.
"Patient safety is our highest priority, and we fully support the decision to pause dosing while we work with the FDA and MHRA to evaluate these findings," said Dr. Stephen Hoge, president of Moderna, on a call with analysts Wednesday morning. "We remain confident in the underlying mRNA platform and expect to provide a path forward in the coming weeks."
Eleven cases of cardiac inflammation were reported across the trial cohort, with nine of those occurring in male participants under 40, according to Dr. Peter Marks, director of the FDA's Center for Biologics Evaluation and Research. "We are asking Moderna to submit additional cardiac MRI data, troponin trends, and a revised risk-benefit analysis before any further dosing occurs," Dr. Marks said in a briefing Wednesday afternoon. "This is a precautionary step, not a determination that the vaccine is unsafe." Dr. Nicole Lurie, Executive Director for Preparedness and Response at the Coalition for Epidemic Preparedness Innovations (CEPI), called the pause "a setback, but a responsible one given the stakes of deploying a pandemic countermeasure at scale."
The pause complicates U.S. pandemic preparedness planning amid sustained concern over H5N1 outbreaks in U.S. dairy cattle, which have now spread to 19 states, and sporadic human cases reported since 2024. The Biomedical Advanced Research and Development Authority (BARDA), which has invested $176 million in the program since 2023, said it would defer a planned contract option for an initial 50 million doses for the U.S. Strategic National Stockpile pending resolution of the clinical hold. The Department of Health and Human Services said in a statement that stockpiled egg- and cell-based H5N1 vaccines from Sanofi and CSL Seqirus remain available, and that contingency planning is underway should the Moderna timeline slip beyond the third quarter of 2026, when the company had previously expected to file for full Biologics License Application.
Shares of Moderna fell 8.4% in pre-market trading following the announcement. The company said it expects to provide an updated trial timeline once discussions with regulators conclude.