MITRAL VALVE PROLAPSE; This 51-year-old, male subject was participating in Placebo-Controlled Study to Evaluate the Efficacy, Safety, and Immunogenicity of mRNA-1273 SARS-CoV-2 Vaccine in Adults Aged 18 Years and Older (mRNA-1273-P301) and experienced the adverse event of special interest (AESI) of mitral valve prolapse.    The subject's medical history, as provided by the investigator, included central coronal retinopathy, frequent sinusitis, vasomotor rhinitis, and acne. Concomitant medications reported included pseudoephedrine hydrochloride, colecalciferol, and budesonide.  Prior to event onset, the subject received intramuscular mRNA-1273 (Dose 4, non-study booster) for SARS-CoV-2 vaccination on 23 Jun 2022, approximately 28 days before onset of the event.  On 20 Jul 2022, the subject experienced mitral valve prolapse, and was admitted to the hospital. The subject was presented to emergency department with complaints of intermittent palpitations of the left side of chest. A murmur was discovered on physical examination. Subject was admitted overnight for observation and echocardiogram. Echocardiogram showed normal left ventricular size and function, normal right ventricular size and function, thickened mitral valve with degenerative posterior leaflet prolapse, and moderate regurgitation, with mildly enlarged left atrium. There was also normal diastolic function, no intracardiac masses, and no prior studies for comparison. No treatment was given at this time. Electrocardiogram showed occasional bradycardia and biphasic T waves in lateral leads, with no concern for ischemia. Troponins were negative x 2.  On 21 Jul 2022, the subject was discharged from the hospital, in stable condition.   The subject received all scheduled doses of study drug prior to the event; therefore, action taken with study drug in response to the event was not applicable.   The event, mitral valve prolapse, was reported as not resolved.  The investigator assessed the event, mitral valve prolapse, as not related to study drug and not related to study procedure. The investigator assessed the event as related to non-study booster dose.  Company comment:  This case concerns a 51-year-old, male subject with a medical history of central coronal retinopathy, frequent sinusitis, vasomotor rhinitis, and acne, who experienced the unlisted event of mitral valve prolapse. The event occurred 1 year 10 months 10 days after the first dose of IP, and 9 months 13 days after the third and booster dose. The subject also received 4 th dose (second booster) of study vaccine [Moderna], outside the study, 28 days prior to the event. Supportive diagnostic procedures ruled out ischemia and heart failure. The event was considered as not resolved. The event was considered unrelated to the study drug and related to the non-study booster dose (4th dose) by the investigator. The sponsor considers the event as unrelated to the study drug and to the non-study booster dose (4th dose) with the information provided at this time providing that other conditions from medical history including habits such as smoking, alcohol consumption, blood pressure values, body mass index, etiology for central coronal retinopathy, relevant family history and cardiac evaluation at baseline, suggestive of alternative etiologies for the event, were not provided. The case will be reassessed when more information becomes available.; Reporter's Comments: This case concerns a 51-year-old, male subject with a medical history of central coronal retinopathy, frequent sinusitis, vasomotor rhinitis, and acne, who experienced the unlisted event of mitral valve prolapse. The event occurred 1 year 10 months 10 days after the first dose of IP, and 9 months 13 days after the third and booster dose. The subject also received 4 th dose (second booster) of study vaccine [Moderna], outside the study, 28 days prior to the event. Supportive diagnostic procedures ruled out ischemia and heart failure. The event was considered as not resolved. The event was considered unrelated to the study drug and related to the non-study booster dose (4th dose) by the investigator. The sponsor considers the event as unrelated to the study drug and to the non-study booster dose (4th dose) with the information provided at this time providing that other conditions from medical history including habits such as smoking, alcohol consumption, blood pressure values, body mass index, etiology for central coronal retinopathy, relevant family history and cardiac evaluation at baseline, suggestive of alternative etiologies for the event, were not provided. The case will be reassessed when more information becomes available.