Progressive isolated lower-lip oedema after mRNA COVID-19 booster complicated by methicillin-resistant Staphylococcus aureus superinfection: a necrotizing fasciitis mimic.

Vaccine-related mucocutaneous reactions are usually mild, with atypical presentations and superinfections that can mimic soft-tissue emergencies. A previously healthy 19-year-old man developed isolated lower-lip oedema 3-4 h after an mRNA COVID-19 booster, peaking at 96 h. Initial blood test results were unremarkable, and angioedema therapy (antihistamines and a short course of corticosteroid) was ineffective. On day 12, he re-presented with marked swelling and necrotic slough but no systemic toxicity [Laboratory Risk Indicator for Necrotising Fasciitis (LRINEC) score of 4]. Debridement and histopathology showed mixed neutrophilic and lymphohistiocytic inflammation extending into skeletal muscle. Wound culture grew methicillin-resistant Staphylococcus aureus (MRSA), and a course of co-trimoxazole led to rapid resolution. The chronology, histopathology and treatment response support delayed-type hypersensitivity temporally associated with vaccination, complicated by MRSA impetiginization that mimicked necrotizing fasciitis. In orofacial presentations, LRINEC has limited sensitivity; hence, clinical correlation and early surgical input are pivotal. We outline a pragmatic approach: consider recent vaccination(s) in rapid-onset mucosal swelling, obtain early cultures, escalate to anti-MRSA therapy if deterioration occurs and involve surgical teams promptly when necrosis or functional compromise is present. Persistent isolated labial oedema after mRNA vaccination may additionally reflect non-IgE delayed hypersensitivity with potential for bacterial superinfection.