Acute on chronic hypoxia; diarrhea; headache; nausea; This spontaneous case was reported by a pharmacist (subsequently medically confirmed) and describes the occurrence of HYPOXIA (Acute on chronic hypoxia) in an 83-year-old male patient who received mRNA-1273 (Moderna COVID-19 Vaccine) for COVID-19 vaccination.     The patient's past medical history included Ventricular tachycardia, Arteriosclerosis coronary artery, Hypertensive and COVID-19 virus test positive (Hospitalized with mild hypoxemia at that time) in January 2021. Concurrent medical conditions included Latex allergy, Allergy (pregabalin), Weakness generalised, Dementia, Atrial fibrillation, Type 2 diabetes mellitus, Mixed hyperlipidemia, Depressive disorder, Polyneuropathy in diabetes, Peripheral vascular disease NOS and Cognitive impairment. Concomitant products included ACETYLSALICYLIC ACID (ASPIRIN (E.C.)), AMOXICILLIN, FLUOXETINE, FUROSEMIDE (FUROSEMIDE ACCORD [FUROSEMIDE]), GABAPENTIN (GABAPENTIN "PCD"), MEMANTINE HYDROCHLORIDE (MEMANTINE ABDI), METFORMIN, METOPROLOL TARTRATE, PANTOPRAZOLE, POTASSIUM CHLORIDE, ROSUVASTATIN and ZINC OXIDE for an unknown indication.    On 17-Mar-2021, the patient received second dose of mRNA-1273 (Moderna COVID-19 Vaccine) (Intramuscular) 1 dosage form. On 18-Mar-2021, the patient experienced HYPOXIA (Acute on chronic hypoxia) (seriousness criteria hospitalization and medically significant). The patient was hospitalized from 18-Mar-2021 to 21-Mar-2021 due to DIARRHOEA, HEADACHE, HYPOXIA and NAUSEA. On 21-Mar-2021, HYPOXIA (Acute on chronic hypoxia) had resolved. Not Provided    DIAGNOSTIC RESULTS (normal ranges are provided in parenthesis if available): On 18-Mar-2021, Angiogram: chest clear with no pneumonia or pe (Inconclusive) chest clear with no pneumonia or PE. On 18-Mar-2021, Chest X-ray: chest clear with no pneumonia or pe (Inconclusive) chest clear with no pneumonia or PE. On 18-Mar-2021, Computerised tomogram head: negative (Inconclusive) negative. On 18-Mar-2021, Electrocardiogram: non ischemic (v-paced) (Inconclusive) non ischemic (v-paced). On 18-Mar-2021, N-terminal prohormone brain natriuretic peptide: 3620 pg/ml (Inconclusive) 3620 pg/ml. On 19-Mar-2021, Angiogram: chest clear with no pneumonia or pe (Inconclusive) chest clear with no pneumonia or PE. On 19-Mar-2021, Chest X-ray: chest clear with no pneumonia or pe (Inconclusive) chest clear with no pneumonia or PE. On 19-Mar-2021, Computerised tomogram head: negative (Inconclusive) negative. On 19-Mar-2021, Electrocardiogram: non ischemic (v-paced) (Inconclusive) non ischemic (v-paced).     For mRNA-1273 (Moderna COVID-19 Vaccine) (Intramuscular), the reporter considered HYPOXIA (Acute on chronic hypoxia) to be related.    After the patient received the 2nd vaccine dose on 03/17, he became weak and unable to walk. He also experienced headache, nausea, and diarrhea. On 03/18, he was sent to the  emergency department where we received a diagnosis of acute on chronic hypoxia related to the 2nd dose of COVID-19 vaccine. He required up to 2 L of O2 during the night time. He was given his regular home medications and Lovenox 40 mg SQ QD for DVT prophylaxis.  On 03/21 he was discharged as his SpO2 improved. His hypoxia returned to his baseline level upon discharge.; 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.