Vaccination Versus Hybrid Immunization: Sera with Similar Anti-Rbd Total IG Units Show Different Virus Neutralization Capacity

Assessing the immune status against infectious diseases, such as SARS-CoV-2, is of paramount importance for public health, especially in acute outbreak situations. Anti-receptor-binding domain (RBD) antibody binding assays have emerged as promising tools for assessing virus neutralization and thereby potential correlates of protection. Here, we investigated whether sera with similar levels of anti-RBD total immunoglobulin units would exhibit differential neutralizing capacities comparing vaccinated individuals and those who received hybrid immunization (vaccination post-COVID-19 recovery). A cohort of 36 individuals with comparable anti-RBD Ig binding units (2500-5000 units (U)/ml), as determined by the Elecsys Anti-SARS-CoV-2S (Roche) ELISA, was evaluated. We employed a live virus neutralization assay to assess the virus neutralizing potential of these sera. Notably, despite similar levels of anti-RBD immunoglobulins (2500-5000 U/ml), significant disparities were observed between the two groups. Specifically, sera from individuals with hybrid immunity exhibited neutralization titers of 1280 (8/18), 2560 (8/18), and 5120 (2/18), whereas those exclusively vaccinated showed lower neutralization titers of 640 (12/18) and 1280 (6/18). Our findings suggest that while the presence of anti-RBD immunoglobulins correlates with neutralizing antibody levels, sera with comparable anti-RBD units display a higher proportion of neutralizing antibodies following hybrid immunization compared to vaccination alone. Our study highlights the need for cautious interpretation of serological outcomes from anti-RBD ELISAs when employed as proxy for determining neutralizing antibody levels. It further accentuates the unmatched value of live virus neutralization assays in accurately assessing virus neuralization.