Accelerating Vaccine Adjuvant Screening: Early Follicular Dendritic Cell and Germinal Center B Cell Biomarkers Predict Protective Efficacy

<b>Background</b>: The current assessment method of the protective efficacy of adjuvanted vaccines remains slow and labor-intensive, hindered by prolonged immunization protocols and complex assays. <b>Methods:</b> To overcome this bottleneck, we demonstrate that early segregated cellular biomarkers enable rapid prediction of protection, using a respiratory syncytial virus (RSV) pre-fusion F (pre-F) protein model with diverse adjuvants in mice. <b>Results</b>: We identified that germinal center (GC) B cell responses (Days 7 and 9 post-immunization) strongly aligned with protective efficacy, except for Alum, which achieved MF59-level protection despite lower GC responses. Crucially, follicular dendritic cell (FDC) abundance at day 7 universally predicted protection across all adjuvants, including Alum, drastically shortening discovery time and effort from at least 4-6 weeks to within 1 week. <b>Conclusions:</b> FDCs and GC B cells serve as complementary early biomarkers that accurately forecast vaccine efficacy. This approach could potentially reduce the need for prolonged immunization regimens by cellular profiling on days 7-9, offering a modest step toward streamlining adjuvant selection and informing vaccine design.