Publication date: Mar 25, 2025

BACKGROUND The Bacillus Calmette-Guerin (BCG) vaccine is used to prevent tuberculosis in countries with high rates of tuberculosis. Although the vaccine is generally low risk, there are reports of minor local adverse effects. Rare complications such as abscess formation have been reported in immunocompromised and pediatric patients. However, reports of inoculation-site abscesses are exceedingly rare in the immunocompetent adult population, and there is no clear management plan that has been agreed upon for these cases. We present a case of a 30-year-old immunocompetent woman who developed a Mycobacterium tuberculosis complex abscess following intramuscular injection of the BCG vaccine. CASE REPORT A healthy 30-year-old woman came in for a MMR vaccination but mistakenly received the BCG vaccine intramuscularly in her left deltoid. One week later, she developed pain and swelling at the injection site. A MRI of her shoulder a week later revealed an intramuscular pseudo-lesion. An ultrasound done another 2 weeks later demonstrated an abscess. An acid-fast bacilli smear and culture were done using aspirated fluid and were negative. Pus began to drain from her shoulder days later, which returned positive on a smear and culture for pan-sensitive Mycobacterium tuberculosis complex. She recovered fully following 6 months of anti-tuberculosis medications (rifampicin, isoniazid, pyrazinamide, ethambutol). CONCLUSIONS Development of an abscess secondary to a vaccine should still be considered as a differential diagnosis for vaccine injection-site swelling in immunocompetent adult patients. Healthcare professionals should always confirm the correct type of vaccination and mode of administration before injection to avoid preventable vaccination complications.

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