P2.252: Post-Vaccination Complication: A Rare Case of Group A Streptococcus-Infected Hematoma, Bacteremia, Myositis, and Muscle Necrosis Following Intramuscular Injection in a Pediatric Patient

Introduction: Vaccination-related complications are extremely rare and typically minor; however, severe complications, such as infection can occur. Awareness of these possibilities is essential for taking appropriate precautions. Nonetheless, it is important to emphasize that vaccinations remain extremely safe and are necessary to protect the overall health of pediatric patients.

Case Description: A 16-month-old male presented three weeks after receiving an intramuscular vaccination. His parents reported that he “bled a lot” from the injection site on his right lateral thigh. He experienced intermittent fever from the time of vaccination until his presentation with a right thigh mass. During this period, his primary care physician prescribed a 10-day course of amoxicillin for a presumed ear infection, although he remained otherwise asymptomatic.
One week prior to presentation, his parents observed a worsening hard, round mass with associated tenderness over the previous injection site. Due to persistent fevers, increasing mass size, and difficulty walking, he was taken to the emergency department for evaluation. He was febrile, and laboratory studies revealed leukocytosis (19.8K), thrombocytosis (PLT 911), hemoglobin of 11.5, and elevated inflammatory markers. Ultrasound imaging raised concern for a tumor or abscess, and subsequent MRI demonstrated diffuse myositis and a 4.6 × 4.1 cm infected hematoma.
During surgical wound evacuation, approximately 75% of the vastus lateralis muscle was resected due to significant necrosis. Initial blood and wound cultures were positive for Group A Streptococcus, specifically clindamycin-resistant Streptococcus pyogenes. Following postoperative care, the patient improved and was discharged home on outpatient IV ampicillin.

Discussion: Several factors in the patient's history suggest that the intramuscular vaccination was the likely source of infection. Despite prior oral antibiotic treatment, S. pyogenes was isolated from both blood and wound cultures. The most probable source of contamination was improper skin antisepsis before vaccination, as Group A Streptococcus is a common skin flora organism. A literature review did not identify similar cases in a patient of this age or with this degree of muscle necrosis and myositis following routine intramuscular vaccination.

Conclusion: Vaccination remains one of the safest and most effective medical interventions in pediatric patients, playing a crucial role in both individual and community health. Severe infections following vaccination or intramuscular injection are exceedingly rare, particularly cases progressing to extensive muscle necrosis or bacteremia. However, clinicians should remain vigilant and consider post-vaccination infection in the differential diagnosis for patients presenting with prolonged fever, localized swelling, or difficulty bearing weight.