Antibody Response Dynamics as an Early Predictor of Clinical Outcome in Severe Fever With Thrombocytopenia Syndrome (SFTS): A Retrospective Cohort Study for Prognostic Model Development

ABSTRACT This study investigated the humoral immune response dynamics in patients with severe fever with thrombocytopenia syndrome (SFTS) to evaluate their prognostic value and construct an early warning model. A retrospective cohort of 36 laboratory‐confirmed SFTS patients with ≥ 4 serial antibody measurements out of 153 patients recruited in the study were analyzed. Patients were categorized by outcome into mild, severe, and fatal groups. The results showed that, the fatal group was older, had more comorbidities, and showed no antibody seroconversion during follow‐up, along with a significantly higher day‐7 viral load. Early seroconversion (IgM/IgG ≤ 7 days post‐admission) was associated with shorter recovery time and higher survival compared to delayed seroconversion (> 7 days) or antibody‐deficient status. Antibody seroconversion time was strongly positively correlated with recovery time. Advanced age (≥ 70 years) was a significant risk factor for severe/fatal outcomes and was linked to delayed or absent seroconversion. A synergistic interaction was found between age and day‐7 antibody status. A simple early warning model combining age and day‐7 antibody status demonstrated good predictive performance with an AUC of 0.83. The findings indicate that antibody response timing and advanced age are key prognostic factors in SFTS. The proposed model shows promise for early identification of high‐risk patients in resource‐limited settings.