Recent clinical research suggests that COVID-19 vaccination with mRNA-1273.712 has a minimal impact on sensitive markers of heart injury in younger populations, reinforcing the vaccine’s favorable safety profile. While clinicians have noted a higher risk of myocarditis—a rare inflammation of the heart muscle—among young men following COVID-19 vaccination, new data provide reassuring evidence on the cardiac effects in adolescents and young adults.

In a phase 4, randomized, observer-blind, placebo-controlled crossover trial (ClinicalTrials.gov Identifier: NCT06634797), investigators examined levels of cardiac troponin I (cTnI), a biomarker that can signal myocardial injury, in individuals aged 12 to 30 years. Participants were randomly assigned in a 1:1 ratio to receive mRNA-1273.712 followed by placebo or placebo followed by mRNA-1273.712, with doses spaced 28 days apart. Blood samples were collected at baseline, 3 days, and 28 days after each injection to monitor changes in cTnI. The primary endpoint focused on the percentage of patients with elevated cTnI three days after vaccination.

Among the 1,000 enrolled patients, 981 were included in the analysis. The median age was 18 years, with 53.8% identifying as female and 65.1% as White; most participants were non-Hispanic (83.9%). Approximately 45% of participants were adolescents aged 12 to 17 years. Baseline characteristics were generally balanced across study groups.

The results showed that 98.2% of participants maintained cTnI levels within the normal range throughout the study period, including at the three-day post-injection mark. Only 18 individuals (1.8%) experienced at least one elevated cTnI measurement, 7 of which were already elevated at baseline. Of the remaining elevations, three occurred after vaccination and eight after placebo administration, indicating no clear association with the vaccine.

Statistical analyses confirmed no meaningful differences between the vaccine and placebo groups in the proportion of participants with elevated cTnI at three days post-injection (difference, 0.6%; 95% CI, -0.1 to 1.4). Most elevations appeared later, such as at the 28-day follow-up, and many were linked to recent vigorous physical activity rather than the vaccine itself.

Safety outcomes were similarly reassuring. Adverse events were infrequent, no cases of myocarditis or pericarditis were reported, and no new safety concerns emerged over the course of the study.

Investigators noted that the study’s generalizability is limited, and interpreting cTnI fluctuations in younger populations can be challenging due to age-specific reference data. Nevertheless, they concluded that these findings “support the overall favorable benefit-risk profile of mRNA-1273 vaccination in younger populations, including those at higher risk for vaccine-associated myocarditis.”