A previous report demonstrated the strong association between the presence of antibodies binding to an epitope region from SARS-CoV-2 nucleocapsid, termed Ep9, and COVID-19 disease severity. Patients with anti-Ep9 antibodies (Abs) had hallmarks of original antigenic sin (OAS), including early IgG upregulation and cytokine-associated injury. Thus, the immunological memory of a previous infection was hypothesized to drive formation of suboptimal anti-Ep9 Abs in severe COVID-19 infections. This study identifies a putative original antigen capable of stimulating production of cross-reactive, anti-Ep9 Abs. From bioinformatics analysis, 21 potential original epitope regions were identified. Binding assays with patient blood samples directly show cross-reactivity between Abs binding to Ep9 and only one homologous potential antigen, a sequence derived from the neuraminidase protein of H3N2 Influenza A virus. This cross-reactive binding affinity is highly virus strain specific and sensitive to even single amino acid changes in epitope sequence. The neuraminidase protein is not present in the influenza vaccine, and the anti-Ep9 Abs likely resulted from the widespread influenza infection in 2014. Therefore, OAS from a previous infection could underlie some cases of COVID-19 disease severity and explain the diversity observed in disease outcomes.