Differential SARS-COV-2 Antigen Specificity Of The Humoral Response In Inactivated Virus-Vaccinated, Convalescent, And Breakthrough Subjects

Analytical methods for the differential determination between natural infection with SARS- CoV-2 vs. immunity elicited by vaccination or infection after immunization (breakthrough cases) represent attractive new research venues in the context of the ongoing COVID-19 pandemic caused by Severe Acute Respiratory Syndrome coronavirus 2 (SARS-CoV-2). Herein, we set out to compare humoral responses against several SARS-CoV-2 structural and non-structural proteins in infected unvaccinated (convalescent), vaccinated, as well as vaccinated and infected (breakthrough) individuals. Our results indicate that immunization with an inactivated SARS-CoV-2 vaccine (CoronaVac) induces significantly higher levels of IgG antibodies against the membrane (M) protein of SARS-CoV-2 as compared to convalescent subjects both, after the primary vaccination schedule and after a booster dose. Moreover, we found that CoronaVac-immunized individuals, after receiving a third vaccine shot, display equivalent levels of N-specific IgG antibodies as convalescents subjects. Regarding non-structural viral proteins, for the two viral proteins ORF3a and NSP8, IgG antibodies were produced in more than 50% of the convalescent subjects. Finally, a logistic regression model and a receiver operating characteristic (ROC) analysis show that combined detection of M and N proteins may be useful as a biomarker to differentiate breakthrough cases from vaccinated and convalescent individuals that did not receive prior vaccination. Taken together, these results suggest that multiple SARS-CoV-2 antigens may be used as differential biomarkers for distinguishing natural infection from vaccination.