A 77-year-old man with a past medical history of type 2 diabetes mellitus, peripheral neuropathy, and chronic obstructive pulmonary disease was admitted to the intensive care unit of Bangladesh Medical College Hospital with acute encephalopathy and non-ST segment elevation myocardial infarction (NSTEMI). The patient was on antidiabetic medicine along with H₂ blocker and multivitamins for his existing diseases. The patient's attendant reported that the patient had received his first dose of the Moderna coronavirus disease 2019 (COVID-19) vaccine just 2 days ago. Physical examination revealed that he had a Glasgow Coma Scale of 8/15; a pulse of 106 beats/min; a respiratory rate of 30 breaths/min; oxygen saturation of 80% on room air, which became with 10 L of oxygen and blood pressure of 90/60 mm Hg at the time of admission. During the hospital stay, the patient was treated conservatively with intravenous antibiotics and other necessary medication. Although we have observed the onset of encephalopathy and NSTEMI following COVID vaccination for this patient, we, as healthcare professionals, cannot directly attribute the cause of the complications to the Moderna vaccine without further epidemiological studies with large samples.