Congestive heart failure decompensation; Rapid afib; idiopathic thrombocytopenic purpura; This spontaneous case was reported by a physician (subsequently medically confirmed) and describes the occurrence of CARDIAC FAILURE CONGESTIVE (Congestive heart failure decompensation), IMMUNE THROMBOCYTOPENIA (idiopathic thrombocytopenic purpura) and ATRIAL FIBRILLATION (Rapid afib) in an 86-year-old male patient who received mRNA-1273 (Moderna COVID-19 Vaccine) (batch no. 029A21A) for COVID-19 vaccination.     The patient's past medical history included Congestive heart failure, Disease coronary artery, Atrial fibrillation, Lumbar spinal stenosis, Neuropathy peripheral and Thrombocytopenia. Concurrent medical conditions included Valvular heart disease NOS.    On 12-Mar-2021, the patient received first dose of mRNA-1273 (Moderna COVID-19 Vaccine) (Intramuscular) 1 dosage form. In March 2021, the patient experienced IMMUNE THROMBOCYTOPENIA (idiopathic thrombocytopenic purpura) (seriousness criteria hospitalization prolonged and life threatening). On 28-Mar-2021, the patient experienced CARDIAC FAILURE CONGESTIVE (Congestive heart failure decompensation) (seriousness criteria hospitalization prolonged and life threatening) and ATRIAL FIBRILLATION (Rapid afib) (seriousness criterion hospitalization prolonged). The patient was hospitalized on 28-Mar-2021 due to ATRIAL FIBRILLATION, CARDIAC FAILURE CONGESTIVE and IMMUNE THROMBOCYTOPENIA. At the time of the report, CARDIAC FAILURE CONGESTIVE (Congestive heart failure decompensation) had resolved and IMMUNE THROMBOCYTOPENIA (idiopathic thrombocytopenic purpura) and ATRIAL FIBRILLATION (Rapid afib) was resolving.      DIAGNOSTIC RESULTS (normal ranges are provided in parenthesis if available): In March 2021, Full blood count: Platelet count of 145K. In March 2021, Platelet count: 125,000 (Low) 125,000. On 28-Mar-2021, Biopsy bone: itp (abnormal) Confirmed ITP. On 28-Mar-2021, Platelet count: 5000 (Low) 5000.     The action taken with mRNA-1273 (Moderna COVID-19 Vaccine) (Intramuscular) was unknown.   For mRNA-1273 (Moderna COVID-19 Vaccine) (Intramuscular), the reporter did not provide any causality assessments.   Concomitant product use was not provided. On 28-Mar-2021, the patient was admitted to the hospital for congestive heart failure. Upon admission, the patient was found to have a very low platelet count of 5000. His platelet count the previous week (exact date not provided) was 125,000.  The patient did not respond to standard of care such as steroids and intravenous (IV) immune globulin. After multiple treatments and additional interventions, his platelets did start to come up.  A bone biopsy confirmed that the patient had developed immune thrombocytopenic purpura (ITP).  He had no history of having ITP.  It as noted the patient had not other recent medication changes other than Covid-19 vaccination. The patient was still recovering at the time of this report.   Most recent FOLLOW-UP information incorporated above includes: On 23-Apr-2021: Email following the call.; Sender's Comments: Based on the current available information and temporal association between the use of the product and the start date of the events, a causal relationship cannot be excluded. Further information has been requested.