Expanding SARS-CoV-2 antigen targets beyond spike using a baculovirus expression system.

The ongoing antigenic evolution of SARS-CoV-2, particularly within the spike glycoprotein, threatens the long-term efficacy of spike-focused vaccines and serodiagnostics. While most authorized COVID-19 vaccines exclusively target the spike protein, growing evidence underscores multivalent strategies incorporating conserved viral antigens. Here, we employed a baculovirus expression vector system (BEVS) to co-express all four structural proteins, including spike, nucleocapsid, membrane, and envelope, in Sf9 insect cells. Surface-displayed antigens were used in a cell-based ELISA to profile IgG responses in convalescent sera. All antigens elicited detectable antibody binding, with nucleocapsid and membrane proteins provoking significantly stronger responses than spike (p < 0.001), and envelope protein showing intermediate reactivity. Statistical analyses revealed distinct patterns of antigenic reactivity. These findings validate the utility of BEVS for multiplex antigen presentation and highlight the immunological and diagnostic value of conserved non-spike antigens. Combined, this study advocates multivalent vaccine strategies and refines serodiagnostics by leveraging broader antigenic targets to counter immune escape.