This case was reported in a literature article and described the occurrence of acute appendicitis in a 18-day-old male patient who received Hepatitis B vaccine. Concurrent medical conditions included thrush (at ten days old and was managed with nystatin). Concomitant products included nystatin. On an unknown date, the patient received Hepatitis B vaccine at an unknown dose. On an unknown date, 18 days after receiving Hepatitis B vaccine, the patient experienced acute appendicitis (serious criteria death, hospitalization and GSK medically significant), appendicitis perforated (serious criteria death, hospitalization and GSK medically significant), serositis (serious criteria death and hospitalization), megacolon  (serious criteria death, hospitalization and GSK medically significant), fever neonatal (serious criteria hospitalization and GSK medically significant), pulseless electrical activity (serious criteria hospitalization and GSK medically significant), hypokalemia (serious criteria hospitalization and GSK medically significant), obtundation (serious criteria hospitalization and GSK medically significant), hypoxia (serious criteria hospitalization and GSK medically significant), metabolic acidosis (serious criteria hospitalization), feeding disorder (serious criteria hospitalization), abdominal tenderness (serious criteria hospitalization), leukocytosis (serious criteria hospitalization), abdominal distension (serious criteria hospitalization), edema (serious criteria hospitalization), anisocoria (serious criteria hospitalization), contusion of trunk (serious criteria hospitalization), absent reflex (serious criteria hospitalization), movements reduced (serious criteria hospitalization), distended bowel (serious criteria hospitalization), hypoglycemia (serious criteria hospitalization), wide complex tachycardia (serious criteria hospitalization), irritability (serious criteria hospitalization) and hypotension (serious criteria hospitalization). The patient was treated with antibiotics NOS. On an unknown date, the outcome of the acute appendicitis, appendicitis perforated, serositis and megacolon were fatal and the outcome of the fever neonatal, pulseless electrical activity, hypokalemia, obtundation, hypoxia, metabolic acidosis, feeding disorder, abdominal tenderness, leukocytosis, abdominal distension, edema, anisocoria, contusion of trunk, absent reflex, movements reduced, distended bowel, hypoglycemia, wide complex tachycardia, irritability and hypotension were unknown. The reported cause of death was appendicitis. An autopsy was performed. The autopsy determined cause of death was serositis, megacolon and appendicitis perforated. The reporter considered the acute appendicitis, appendicitis perforated, serositis, megacolon, fever neonatal, pulseless electrical activity, hypokalemia, obtundation, hypoxia, metabolic acidosis, feeding disorder, abdominal tenderness, leukocytosis, abdominal distension, edema, anisocoria, contusion of trunk, absent reflex, movements reduced, distended bowel, hypoglycemia, wide complex tachycardia, irritability and hypotension to be related to Hepatitis B vaccine. Additional information was provided. This case was reported in a literature article and described the occurrence of acute neonatal appendicitis in a 18-day-old male patient who was vaccinated with unspecified hepatitis-B virus vaccine (manufacturer unknown). The patient was born at full term via spontaneous vaginal delivery following an unremarkable pregnancy. No information on patient's medical history or family history was provided. On an unspecified date shortly after birth, the patient received unspecified hepatitis-B virus vaccine (administration route and site unspecified; dosages unknown; batch number not provided). The patient stayed in the newborn nursery for two days prior to discharge. At 10 days old, the patient was diagnosed with thrush and was managed with nystatin. On an unspecified date, 19 days after vaccination, the patient presented at an emergency department with 1-day complaints of increased fussiness and difficulty feeding and was noted to have slightly distended and tender abdomen (a clinical picture of a possible small bowel obstruction). A septic workup was performed and included a complete blood count (CBC) (revealed leukocytosis of 28.5k/UL), a negative blood culture, a lumbar puncture (clear fluid with a negative Gram stain), urinalysis (25mg/dL protein with negative nitrite and leukocyte esterase), and elevated C-Reactive Protein (CRP) (194.6mg/L). The patient was subsequently started on antibiotics for neonatal fever. During his three days of hospitalization, the clinic team requested a transfer to the paediatric Intensive Care Unit (ICU) due to worsening of abdominal distension, increasing white count to 43.81k/UL, and suspicions of small bowel obstruction. The patient further deteriorated during transfer and was significantly obtunded on arrival to the paediatric ICU. He was grossly oedematous, with abdominal distension, anisocoria, bruising along the right flank, absence of reflexes, and minimal spontaneous movement. Radiographic studies performed included a Kidneys, Ureter, and Bladder (KUB) (showed absence of air in the rectum, oedema of the bowel walls, but no free air on cross table film), an abdominal X-ray (showed a mild-to-moderate degree of gaseous distension of the bowel), and a chest X-ray (showed bilateral pulmonary opacities). The patient was acutely managed for hypoxia associated with severe metabolic acidosis, hypokalaemia, hypotension, and hypoglycemia. He subsequently had several episodes of recurrent wide complex tachycardia and pulseless electrical activity. After multiple resuscitations, the patient was pronounced dead several hours after the transfer. An autopsy was performed and the most significant findings were gangrenous appendicitis with evidence of rupture and marked acute serositis identified in the rectal serosa and focally in the mesenteric peritoneum. There was dilation of the transverse colon and proximal descending colon but no obstruction of the bowel was identified. This case has been considered serious due to death/hospitalisation. The author did not comment on the relationship between the event of acute neonatal appendicitis and unspecified hepatitis-B virus vaccine. The authors stated, "Acute neonatal appendicitis is more common in males, with up to a half of all reported cases involving premature infants. The different factors attributing to the low incidence include funnel-shaped appendix with a wide opening into the cecum, soft liquid diet, lack of faecalith, recumbent posture, and the presumed infrequent occurrence of viral-induced lymphatic hyperplasia in the periappendiceal region. Perforation plays a significant role in determining the prognosis. Due to the delayed diagnosis and management, the incidence of perforation and subsequent peritonitis is high in neonatal appendicitis. Other factors contributing to the increased susceptibility to perforation in this population include the thin appendiceal wall, a nondistensible cecum, a relatively small omentum insufficient to wall of infection, and a small capacity of the abdominal cavity, resulting in easy dissemination of infection. Similar to this case, acute appendicitis remains a diagnostic challenge and, in the majority of cases, is discovered only on postmortem examination. A high index of clinical suspicion and meticulous analysis of clinical features can lead to early diagnosis and more timely surgical intervention to reduce the associated high mortality rate".