The U.S. Food and Drug Administration has strengthened safety warnings on Pfizer’s Comirnaty and Moderna’s Spikevax COVID-19 vaccines, citing additional evidence that the mRNA products can trigger myocarditis and pericarditis, particularly in young males. In a notice published this week, the agency instructed both manufacturers to update their Prescribing Information, patient Fact Sheets and Adverse Reactions sections to highlight the latest data on incidence, severity and persistence of vaccine-associated cardiac inflammation. The new language builds on an earlier safety communication issued 25 June 2024 and incorporates findings from an FDA-funded longitudinal MRI study that tracked roughly 300 myocarditis cases. That study found that 60 percent of affected patients continued to show late gadolinium enhancement—a marker of cardiac injury—five months after vaccination. Surveillance data covering the 2023-24 immunisation season put the unadjusted incidence rate at about 27 myocarditis and/or pericarditis cases per million doses in males aged 12-24, the demographic judged to be at greatest risk. Pfizer and Moderna have also been ordered to maintain ongoing follow-up studies to clarify the long-term cardiac outcomes of vaccine-linked myocarditis. The FDA said the additional warnings aim to ensure clinicians and vaccine recipients “are fully informed of both the known and the potential risks” while immunisation campaigns continue.