Abstract

Persistent swelling at a vaccine injection site is an uncommon occurrence which is rarely biopsied. Here, we present a 37-month-old male with a history of mild atopic dermatitis and pityriasis rosea who presented with an enlarging nodule in his left deltoid following vaccinations at eight months of age. DTaP-IPV-HepB (Diphtheria, Tetanus, Pertussis, Poliovirus, hepatitis B), Pneumococcal conjugate, and Haemophilus influenzae type B vaccines were administered at the site of the lesion. Physical examination revealed a 5-millimeter firm subcutaneous papule with some hyperpigmentation of the overlying skin. Ultrasound on the lesion reveals a lobulated heterogeneously hypoechoic structure. The lesion was excised. Histological examination demonstrated subcutaneous fibrosis with patchy lymphohistiocytic inflammation with a few lymphoid follicles and focal palisaded granuloma around a zone of altered collagen. Rare purplish non-polarizable material was identified within this altered collagen. The histiocytes contained granular amphophilic cytoplasm. Grocott methenamine silver stain was negative for fungal organisms. Ziehl-Neelsen and Fite stains were negative for mycobacteria. The histopathologic features were consistent with a persistent vaccine reaction, perhaps to aluminum adjuvants. These findings were in keeping with limited previous histopathologic studies of this phenomenon. While benign, injection-site reactions can have a wide spectrum of histological features and may mimic other disease entities, including infection, lupus profundus, and lymphoma. Careful histological examination and clinical correlations are imperative for the accurate diagnosis of this condition.