This is a literature case, initially received on 18-Dec-2018, concerning a 4-year-old, male child patient. The patient's historical condition included mild, intermittent asthma and was otherwise healthy. On an unspecified date, the patient was administered INN Flu Vaccine Seasonal [dose, route of administration, anatomical location, batch number, expiry date, trade name and manufacturer: not reported] for an unknown indication. On an unspecified date after vaccination (reported as 5 months ago), the patient developed high fever and right-sided chest pain of 2 days duration. On an unspecified date, the patient was hospitalized. Upon admission, his body temperature was 103.7-degree F, heart rate was 160/minute, respiratory rate was 32 per minute, blood pressure was 95/54 mmHg, and oxygen saturation was 95 percent on room air. The patient was alert, interactive, in moderate respiratory distress with subcostal retractions and no air entry on the right side. Nasopharyngeal swab was positive for influenza B (explicitly coded as vaccination failure). Chest X-ray showed right lower lobe pneumonia with pleural effusion and a normal-sized cardiac silhouette. He was placed on high-flow nasal cannula, and intravenous ceftriaxone and oral oseltamivir were initiated. A right pleural pigtail catheter was inserted and drained 130 ml of serosanguineous fluid. CT scan showed a moderate to large loculated right-sided pleural effusion with complete consolidation of the right lung. The patient underwent video-assisted thoracoscopic surgery with pleural decortication and adhesiolysis. Pleural fluid culture grew methicillin-resistant S. aureus, and vancomycin and rifampin were added to the antibiotic regimen. Clinical improvement did not occur as expected after video-assisted thoracoscopic surgery, with persistence of high-grade fevers and general malaise. He developed generalised oedema, diffuse abdominal pain, and feeding intolerance. Plain X-ray of the abdomen showed multiple dilated loops of small bowel suggestive of subacute obstruction. Abdominal ultrasound revealed small amount of ascites and oedema surrounding the gall bladder. Persistent tachycardia and increase in respiratory distress warranted escalation of non-invasive respiratory support. A chest X-ray on day 6 of hospitalisation showed enlarged cardiac silhouette, and echocardiogram showed a moderate circumferential pericardial effusion with stranding. Interventricular bounce was present, suggesting haemodynamically significant effects on diastolic function. Pericardial window surgery with drainage of effusion yielded 250 ml of yellow, cloudy fluid. The pericardial fluid did not grow any organism in culture. Multiple blood cultures were also negative. There was significant clinical improvement, and the patient was discharged home after receiving 4 weeks of intravenous vancomycin and rifampin. The patient was doing well on follow-up. The outcome was recovered. The reporter assessed the case as serious (hospitalization). Reporter's Comments: A 4-year-old male patient was diagnosed with influenza B infection (considered as vaccination failure), community acquired methicillin-resistant Staphylococcus aureus co-infection, purulent pericarditis, right lower lobe pneumonia, empyema, pericardial effusion, ascites, small bowel obstruction, gallbladder edema, generalised edema and diastolic dysfunction, 5 months after receipt of INN flu vaccine seasonal. The diagnoses were supported by abnormal investigations which included influenza B test positive, chest X ray, CT scan of chest, ultrasound abdomen, pleural fluid culture positive and ECG. Considering the fact that influenza vaccines do not have 100% effectiveness and a protective immune response may not be elicited in all vaccine recipients, the event influenza and vaccination failure assessed as related to suspect vaccine. The event purulent pericarditis and empyema (proven methicillin-resistant Staphylococcus aureus infection) are infective in origin, hence the role of inactivated suspect vaccine is ruled out. The events right lower lobe pneumonia, pericardial effusion, ascites, generalised edema, gallbladder edema and diastolic dysfunction are the complications of underlying events empyema and purulent pericarditis. Hence, assessed as not related to suspect vaccine. The event small bowel obstruction, assessed as not related considering biological implausibility with the suspect vaccine and multiple underlying events are considered as confounding factors. The company assessed the events influenza B virus infection, vaccination failure, purulent pericarditis, pneumonia lobar, methicillin-resistant Staphylococcus aureus infection, bowel obstruction, gallbladder oedema, pericardial effusion and ascites as serious (medically significant).