Johnston MS, Galan A, Watsky KL, et al. - In this retrospective case series study involving 16 patients (median [range] age, 38 [25-89] years; 13 [81%] women), researchers sought to characterize the course of localized cutaneous injection-site reactions to the Moderna COVID-19 vaccine, subsequent reactions to the second vaccine dose, and to describe the findings of histopathologic examination of the reaction. This investigation was carried out at Yale New Haven Hospital in New Haven, Connecticut, to evaluate clinical and histopathologic features of injection-site reactions to COVID-19 vaccines. This case series study's clinical and histopathologic findings suggest that the localized injection-site reactions to the Moderna COVID-19 vaccine are a delayed hypersensitivity reaction. These reactions may occur sooner after the second dose, but they are self-limiting and are not linked to serious vaccine side effects. These delayed hypersensitivity reactions (dubbed "COVID arm"), unlike immediate hypersensitivity reactions (eg, anaphylaxis, urticaria), are not a contraindication to subsequent vaccination.