shortness of breath/ it was determined that he had developed pericardial effusion; irregular heart beat; atrial fibrillation; This spontaneous case was reported by a consumer and describes the occurrence of PERICARDIAL EFFUSION (shortness of breath/ it was determined that he had developed pericardial effusion), ARRHYTHMIA (irregular heart beat) and ATRIAL FIBRILLATION (atrial fibrillation) in a 71-year-old male patient who received mRNA-1273 BIVALENT .222 (MODERNA COVID-19 VACCINE, BIVALENT (ORIGINAL AND OMICRON BA.4/BA.5)) (batch no. AS7643E) for COVID-19 prophylaxis.     Previously administered products included for COVID-19 vaccination: Spikevax (1st Dose: Lot # 028L20A) on 22-Jan-2021, Spikevax (2nd Dose: Lot # 013M20A) on 17-Feb-2021, Spikevax (3rd Dose: Lot # 071F21A) on 28-Oct-2021 and SPIKEVAX BIVALENT ORIGINAL/OMICRON BA.4-5 (5th Dose:  Lot # AS7143C) on 16-Sep-2022; for COVID-19 prophylaxis: Pfizer (4th Dose: Lot # FM9492) on 29-Apr-2022. Past adverse reactions to the above products included No adverse event with Pfizer, SPIKEVAX BIVALENT ORIGINAL/OMICRON BA.4-5, Spikevax, Spikevax and Spikevax. Concurrent medical conditions included Seasonal allergy and Dust allergy.    On 16-Jun-2023, the patient received sixth dose of mRNA-1273 BIVALENT .222 (MODERNA COVID-19 VACCINE, BIVALENT (ORIGINAL AND OMICRON BA.4/BA.5)) (Intramuscular use) .5 milliliter. On 19-Jun-2023, the patient experienced PERICARDIAL EFFUSION (shortness of breath/ it was determined that he had developed pericardial effusion) (seriousness criteria hospitalization and medically significant) and ATRIAL FIBRILLATION (atrial fibrillation) (seriousness criterion medically significant). In August 2023, the patient experienced ARRHYTHMIA (irregular heart beat) (seriousness criterion medically significant). The patient was hospitalized on 17-Jul-2023 due to PERICARDIAL EFFUSION. The patient was treated with APIXABAN (ELIQUIS) ongoing since an unknown date for Arrhythmia and Atrial fibrillation, at an unspecified dose and frequency; IBUPROFEN at a dose of high dose for 30 days; COLCHICINE at a dose of for 3 months and METOPROLOL SUCCINATE ongoing since an unknown date for Heartbeats irregular and Atrial fibrillation, at an unspecified dose and frequency. At the time of the report, PERICARDIAL EFFUSION (shortness of breath/ it was determined that he had developed pericardial effusion), ARRHYTHMIA (irregular heart beat) and ATRIAL FIBRILLATION (atrial fibrillation) had not resolved.      DIAGNOSTIC RESULTS (normal ranges are provided in parenthesis if available): On 19-Jul-2023, Laboratory test: pericardial effusion. On an unknown date, Cardiac monitoring: Detected 19 episodes in 2 weeks. On an unknown date, Heart rate: irregular heartbeat. On an unknown date, Laboratory test: normal range.     For mRNA-1273 BIVALENT .222 (MODERNA COVID-19 VACCINE, BIVALENT (ORIGINAL AND OMICRON BA.4/BA.5)) (Intramuscular use), the reporter did not provide any causality assessments.   Concomitant medication information was not reported.  The patient reported that within 5 days of receiving the vaccine, he started experiencing shortness of breath. He was unable to get an appointment with his doctor as they were on vacation at the time, so he didn't pursue it further. He started experiencing worsening shortness of breath, even on walks that normally would be okay for him. The shortness of breath got worse before it got severe. On 17-Jul-2023, his shortness of breath became very severe, and he had to be admitted to the hospital. Patient had chest pain. He also reported symptoms of dyspnoea (Difficult/labored breathing),dyspnoea exertional, palpitations and painful respiration. His symptoms were still ongoing. Several tests were done at the hospital, and it was determined that he had developed pericardial effusion. It was unknown what had caused the effusion as all other tests were negative. Patient had 35-40 tests done altogether. He says that in late AUG 2023, he developed an irregular heartbeat and atrial fibrillation. His cardiologist informed that this was a reflection of the effusion that he had developed. He was currently being treated for the irregular heartbeat and atrial fibrillation with Eliquis and Metoprolol succinate. Since he has started the medications, he hasn't had irregular heartbeats and so he would be re-tested to see how those symptoms currently are. He stated that when he was tested for it, he was put on a heart monitor that had detected 19 episodes in 2 weeks. the patient called to find out whether he should go for another dose of the COVID-19 vaccine or not at this time, given he had these symptoms. He received mixed reviews from his doctors. His cardiologist thinks that the pericardial effusion was a coincidence, whereas his primary care physician thinks it could have been a possible side effect.  Company Comment This is a spontaneous case concerning a 71-year-old, female patient with no relevant medical history, who experienced the unexpected serious (hospitalization and medically significant) event of Pericardial effusion and unexpected serious (medically significant) AESI of Atrial fibrillation and Arrhythmia. The events began with dyspnea one day after the administration of Spikevax Bivalent .222 (Original/Omicron BA.4/5) vaccine. The patient was admitted to hospital and Pericardial effusion was confirmed by echo test. Patient also developed Atrial fibrillation and Arrhythmia treated with apixaban, metoprolol, ibuprofen and colchicine. No further information on clinical course, investigations such as troponin levels was available in the report. The benefit-risk relationship of Spikevax Bivalent .222 (Original/Omicron BA.4/5) Vaccine is not affected by this report.   Most recent FOLLOW-UP information incorporated above includes: On 27-Oct-2023: Live follow-up received on 27 Oct 2023 with significant information. New reference added, treatment medication lab and narrative updated.