This 24-year-old female subject was participating in a Phase 3b, Open-Label, Safety and Immunogenicity Study of SARS-CoV-2 mRNA-1273 Vaccine in Adult Solid Organ Transplant Recipients and Healthy Controls (mRNA-1273-P304) and experienced the adverse event of special interest of probable myocarditis. The subject's medical history, as provided by the investigator, included liver transplant, portal hypertension, autoimmune hepatitis, autoimmune hepatitis cirrhosis, depression, panic attacks, bilateral foot drop, left wrist injury, mountain biking accident, telangiectasias, acute fever - 3 episodes, low potassium levels (hypokalemia), generalized anxiety disorder, common peroneal neuropathic pain, ascites, thrombocytopenia, and elbow injury. Concomitant medications reported included tacrolimus, ursodeoxycholic acid, paracetamol, azathioprine, calcium carbonate, colecalciferol, citalopram, hydroxyprogesterone caproate, and magnesium oxide. The subject received intramuscular mRNA-1273 (Dose 1) for SARS-CoV-2 vaccination on 23 Sep 2021, on the same day as onset of the event. On 23 Sep 2021, the subject experienced probable myocarditis. On 24 Sep 2021, she reported to the emergency department (ED) complaining of chest pain that started the previous day and had continued and intensified. When the chest pain was intense, the subject stated feeling shortness of breath. She stated the chest pain was intermittent and reported intense pressure lasting 5-10 minutes upon occurrence. The subject reported headache, muscle aches and fever of 101.5 and 100.8 F.(most recent). She received a 12-lead electrocardiogram (EKG), which was reported as normal sinus rhythm. The subject left the ED due to this normal result under the assumption she was experiencing anxiety. EKG was not repeated. The EKG was compared with an EKG from 16 Dec 2020, and non-specific anterior T wave abnormality became evident in anterior leads. Per the principal investigator (PI), it was probable myocarditis based on new/updated EKG results, and no labs were done. The subject was told by the nursing staff that her EKG was normal and the subject waited for more than four hours to see a physician. However, she was not seen by any doctor or advanced practice providers, so she left without being seen. There was no discharge diagnosis in the ED. Per subject, she left because she felt that she was not being taken seriously by emergency room staff since her EKG came back normal and she has a history of anxiety, which was well-managed on citalopram and she had no symptoms of chest pain or shortness of breath after. It was reported that the event was not a hypersensitivity reaction. Her symptoms might have been anxiety-related and were very transient. The PI's clinical impression was that myocarditis was unlikely. However, the PI classified the event as probable based on his interpretation of the criteria as defined in the protocol. No treatment was given for the event. On 30 Sep 2021, the PI (principal investigator) saw the subject and all the symptoms had resolved. On 21 Oct 2021, the subject was seen again and she was well. The subject received the only scheduled dose of study drug prior to the event; therefore, action taken with the drug in response to the event was not applicable. The event, probable myocarditis, was reported as resolved on 28 Sep 2021. The investigator assessed the event, probable myocarditis, as related to study drug and not related to study procedure. The reasons for assessing the event as potentially related to vaccination were due to close time relationship to vaccination, absence of other clear etiology, and known prior (rare) association of mRNA vaccination with myocarditis, including higher risk for younger patients. Follow-up received on 09 Nov 2021 included updated event start date, updated action taken and event details. Follow-up received on 07 Dec 2021 included updated clarification regarding PI (principal investigator) assessment of event term. Follow-up received on 14 Dec 2021 included PI (principal investigator) details and impression of case. Follow-up received on 20 Jan 2022 included updated emergency room visit details, concomitant medications, medical history, confirmation of no treatment, and investigator rationale for study drug causality assessment. Follow-up received on 02 Sep 2022 included additional medical history and concomitant medication information. Company Comment: This case concerns a 24-year-old female subject with a medical history of autoimmune hepatitis, autoimmune hepatitis cirrhosis, portal hypertension, liver transplant, telangiectasias, acute fever - 3 episodes, low potassium levels (hypokalemia), and anxiety, among others, who experienced the non-serious adverse event of special interest (AESI) of probable myocarditis. The event occurred on the same day after the single dose of mRNA-1273. The PI's (principal investigator) clinical impression was that myocarditis was unlikely as EKG changes were non-specific, and her symptoms were very transient and might have been anxiety related. No other tests were performed, or treatment applied. The event was reported as resolved. The event was considered related to the study vaccine in agreement with the Investigator based on close temporal association.