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Cutaneous polyarteritis nodosa following ChAdOx1 nCoV-19 vaccination Dear Editor, Cutaneous small-vessel vasculitis following COVID-19 vaccination has been reported. Herein, we report a case of cutaneous polyarteritis nodosa (CPAN) involving medium-sized arteries post-ChAdOx1 nCoV-19 vaccination (Oxford-Astrazeneca: AZD1222). A 52-year-old otherwise healthy Taiwanese woman presented with tender erythematous subcutaneous nodules on bilateral lower legs 1 month after the first ChAdOx1 nCoV-19 vaccination (Fig. 1a). The exacerbation occurred after the reexposure to the second dose. She denied fever, myalgia, arthralgia, body weight loss, cough, sore throat, or abdominal pain. She had no known personal and family history of allergy and autoimmune diseases. Recent drug exposure was denied. Laboratory data of complete blood counts, liver enzymes, creatinine, C3, C4, erythrocyte sedimentation rate, C-reactive protein, and rheumatoid factor were within normal limits. Antinuclear antibodies, antineutrophil cytoplasmic antibodies (p-ANCA, c-ANCA), and anti-SSA/SSB antibodies were not detected. Hepatitis B surface antigen and hepatitis C antibody were nonreactive. The result of SARS-CoV-2 rapid antigen test, antistreptolysin O antibody, and QuantiFERON-TB Gold test were negative. Urinalysis showed the absence of hematuria and dysmorphic red blood cells. Histopathologically, there was fibrinoid necrosis with neutrophilic infiltration, nuclear dust, and erythrocyte extravasation around the medium-sized arteries in the junction of dermis and subcutis (Fig. 1b). By typical cutaneous manifestation, histopathological finding, and the exclusion of systemic involvement, the diagnosis of CPAN was made. Treatment with daily oral prednisolone 20 mg and weekly methotrexate 7.5 mg for 2 weeks successfully eliminated the skin lesions. Cutaneous polyarteritis nodosa is a rare disease characterized by medium-sized necrotizing vasculitis of arteries limited to the skin with a favorable prognosis. The cutaneous manifestations include livedo reticularis, tender subcutaneous nodules, and cutaneous ulcerations primarily on the legs. The etiology of CPAN may be associated with the infections, inflammatory bowel disease, and the use of minocycline. The aforementioned etiologies were excluded in our case. The only remarkable history was vaccination. The ChAdOx1 nCoV-19 vaccine (AZD1222) is a nonreplicating adenoviral vector vaccine which encodes the structural surface glycoprotein antigen (spike protein) of SARS-CoV-2. The rare cutaneous adverse events reported to be associated with ChAdOx1 nCoV-19 vaccine include injection-site reactions, urticaria and/or angioedema, local delayed type hypersensitivity reaction, pityriasis rosea-like eruptions, and leukocytoclastic vasculitis. CPAN is never known to be associated with COVID-19 vaccines despite other vasculitides having been reported. Historically, polyarteritis nodosa (PAN), including its cutaneous variant, had been established as the rare adverse event of hepatitis B vaccine and influenza vaccine. A peak time to PAN onset was reported to be 2 weeks postvaccination. The pathomechanism of vaccine-related PAN was uncertain, but the hypothesis of molecular mimicry theory had been advocated. The antigen of the vaccine resembles a host antigen as in the case of infection and consequently triggers an immune response that attacks the invading pathogen as well as the host Figure 1 The dermatologic presentation and histology of the cutaneous polyarteritis nodosa in a 52-year-old female 1 month after receiving the ChAdOx1 nCoV-19 vaccination. (a) Scattered ill-defined, nonblanchable, erythematous, tender subcutaneous nodules on bilateral lower legs. (b) Histology revealed a typical polyarteritis nodosa involving medium-sized muscular arteries without evidence of septal panniculitis. (Hematoxylin–eosin stain, 940)