ABSTRACT
Introduction: The most common side effects were mild pain at the injection site and fever after the COVID-19 vaccination. A retroperitoneal abscess is a rare disorder with a deceptive onset and difficult diagnosis. It has various reasons and is related to a high mortality rate. Case presentation: A 29-year-old man with a recent history of first-dose Covid-19 vaccination, was referred for dyspnea, chest, and abdominal pain. Chest imaging revealed a lung abscess evacuated to pleural space. Left posterolateral thoracotomy surgery was done. Post-operation abdominopelvic imaging revealed increased fat stranding and fluid collection, suggesting retroperitoneal infection and abscess formation and the patient underwent drainage. Clinical discussion: Common side effects after COVID-19 vaccination were mild and expectable without hospitalization. But in our case, a rare complicated side effect was seen. Conclusion: Uncommon side effects should be observed to recognize whether they are related to the vaccine or not.