Among 777 participants who received a COVID-19 mRNA-1273 booster vaccination, 40 (5.1%; 95% confidence interval [CI] 3.7–7.0%) had an elevated high-sensitivity cardiac troponin T (hs-cTnT) concentration on day 3, of whom 22 (2.8% [95% CI 1.7–4.3%]) were adjudicated as mRNA-1273 vaccine-associated myocardial injury. Females (n = 20, 3.7% [95% CI 2.3–5.7%]) had a higher incidence than males (n = 2, 0.8% [95% CI 0.1–3.0%]). Hs-cTnT elevations were mild and decreased from day 3 to the follow-up visit in all but one case. No participants developed major adverse cardiac events within 30 days. Participants with mRNA-1273 vaccine-associated myocardial injury had comparable systemic reactogenicity, but lower concentrations of interferon (IFN)-λ1 (IL-29) and granulocyte-macrophage colony-stimulating factor (GM-CSF) versus participants without vaccine-associated myocardial injury.