Surrogate virus neutralization test as a scalable and reliable tool for immune surveillance in booster-vaccinated...

With waning vaccine-induced immunity and the continued emergence of immune-evasive SARS-CoV-2 variants, booster vaccination has become essential for sustaining population-level protection. However, scalable and reliable serological tools for monitoring post-booster humoral immunity across different vaccine platforms remain insufficiently evaluated. We conducted a cross-sectional study and a booster cohort study to systematically compare the performance and concordance of surrogate virus neutralization test (sVNT), pseudovirus neutralization test (pVNT), and binding IgG antibody assays. In the cross-sectional analysis, serum samples from 259 individuals collected 1-2 months after a third dose of inactivated vaccine were used to compare sVNT and pVNT. In the booster cohort, paired sera from 288 participants receiving a fourth dose with different vaccine platforms were analyzed to evaluate the relationship between sVNT and IgG responses. Correlation and agreement were assessed using Spearman correlation and Bland-Altman analyses. In the cross-sectional study, sVNT showed strong concordance with pVNT, demonstrated by a very high correlation (r = 0.97) and good agreement in Bland-Altman analysis. In the booster cohort study, IgG antibody levels correlated strongly with sVNT overall (r = 0.91). The correlation strength remained highly consistent across different vaccine platforms, with no statistically significant differences observed. Collectively, these findings demonstrate that sVNT closely reflects functional neutralizing activity, supporting its utility for large-scale immune monitoring. sVNT offers an optimal balance of analytical performance and scalability for high-throughput monitoring without requiring cell-based assays. Consequently, we propose a tiered immune monitoring strategy: utilizing sVNT for broad population-level surveillance and reserving pVNT for targeted, precise functional assessment.