Seroprevalence of Respiratory Syncytial Virus in the antenatal population in British Columbia: A cross-sectional study.

Respiratory Syncytial Virus (RSV) is a major cause of bronchiolitis in children under two. With new RSV prophylactics-maternal vaccines and infant monoclonal antibodies-on the horizon, understanding baseline immunity is essential. In this cross-sectional study, we used a validated electrochemiluminescence assay to measure RSV prefusion F (Pre-F) IgG levels in residual sera collected for routine antenatal screening across British Columbia, Canada (n = 13,119) and infant samples aged 0-18 months (n = 47). Antenatal IgG levels showed no significant age-related variation, while infant antibodies declined markedly from birth to 18 months (GMC: 81,900-670 AU/ml). Using presumed positive and negative samples, we established a seropositivity cutoff of 39,223 AU/ml (AUC: 96 %, sensitivity: 98.3 %, specificity: 87 %). Applying this threshold to antenatal samples from August 2021 to April 2025 revealed consistently high seroprevalence (>92 %) with no seasonal variation. These findings validate a high-throughput assay for RSV immunization strategies.