Patient previously received Moderna COVID-19 vaccines on 4/19/2021 and 5/17/2021. COVID-19 vaccine (Pfizer BioNTech for 12 years old and above; Lot # FK9729) administered at 1:15pm on 02/04/2022. Patient walked out of the nurse's room after vaccination and sat down in the hallway to be observed for 15 minutes. Patient was monitored and discharged by clinical staff after observation without incident. Nursing discharge staff reported to this nurse "that at time of discharge patient did not have any complaints, was alert and talkative".  At 1:35pm patient returned back to the clinical area and reported to nursing staff that after he was let go, he and his sister were walking out of the facility to the parking lot and he began to feel a chest tightness and noticed the left side of his upper chest looked larger than the right side. Patient was observed to be slightly pale and very talkative, expressed concerns that his left side of chest was larger than that of his right side. Patient was brought inside room 218, observed walking with a steady gait and sat down on the chair. Patient was coherent and not in any respiratory distress. Assessed patient further, denied any chest pain, left arm pain, shortness of breath, dizziness, blurred vision, headache, or nausea. Upper chest wall visually assessed and symmetrical with no swelling noted at that time.  Patient vital signs checked at 1:36 pm - Blood Pressure 154/82 mmHg; Pulse 106 bpm; Respirations 18; Oxygen Saturation via pulse oximeter 99%. Patient was alert and oriented to name, time and place and in no respiratory distress. Patient reported that he was diagnosed with anxiety about two years ago and is currently taking Prozac and Ashwagandha Supplement. Patient reported "my last meal was last night about 9 and it was pasta with shrimp". Nurse Manager notified of the event and recommended to continue monitoring and to notify her of any changes. Patient was provided with juice, water and a granola bar to eat. Vital signs re-checked at 1:38pm - Blood pressure 137/84 mmHg; Pulse 130 bpm; Respirations 18; Oxygen Saturation via pulse oximeter 100%. At 1:45pm patient reported that he was no longer experiencing the chest tightness and he felt calmer. Informed patient that he was to be monitored an additional 30 minutes before disposition/discharge. Patient was observed to regain his normal color, and in no respiratory distress, sitting upright, and talking. He was alert and oriented to name, time, and place, denied having any chest tightness, chest pain, bilateral arm pain, back pain, stomach pain, headache, nausea, blurred vision, or dizziness. At 1:47 pm, vital signs re-checked: Blood pressure 149/65 mmHg; Oral Temperature: 97.7 °F; Pulse 92 bpm; Oxygen Saturation via pulse oximeter 100%. Patient stated, "I feel fine and don't have any chest tightness anymore". Vital signs re-checked at 2:10 pm: Blood pressure 136/74mmHg; Pulse 65 bpm; Oxygen Saturation via pulse oximeter 98%; Respirations 18. Advised client that if any chest pain or tightness occurs again, or any trouble breathing, or any other abnormal symptom to immediately call 911 or family to take client to the nearest emergency room. Client verbalized understanding and left the clinic ambulatory, in no distress or pain, with his father and siblings.