Transplacental transfer efficiency and longitudinal dynamics of antibodies against RSV in Chinese children from birth to 8 years: a paired mother-neonate cohort study

Maternal antibodies protect infants from severe respiratory syncytial virus (RSV) infection in early life, but this immunity is short-lived, resulting in increasing infection risk over time. With the approval of the first maternal vaccine and half-life extended monoclonal antibodies (mAb) for infants, understanding transplacental transfer efficiency and RSV antibody dynamics is essential for optimizing immunization strategies. A longitudinal paired mother-neonate cohort study was conducted in southern China from 2013 to 2021, with serum samples collected from mothers at delivery and from neonates at birth, and subsequently at 2, 4, 6, 12, 24, 36 months and 5-8 years of age. All samples were tested for RSV pre-F IgG antibodies using an enzyme-linked immunosorbent assay (ELISA). A total of 695 mother-neonate pairs were enrolled in this study. A strong positive correlation ({rho} = 0.87; P<0.001) was observed between maternal antibody titers at delivery (geometric mean titer [GMT]: 4.74; 95% confidence interval [95% CI]: 4.72-4.76) and neonatal antibody titers at birth (GMT: 4.90; 95% CI: 4.88-4.92). The mean transplacental transfer ratio was estimated to be 1.44 (95% CI: 1.40-1.47). Maternal antibody titers and gestational age at birth were significantly associated with neonatal antibody levels. A ten-fold increase in maternal antibody titers was associated with a 5.46-fold increase in neonatal antibody titers (95% CI: 4.89 - 6.08; P<0.001). Compared with full-term birth (gestational age 37-42 weeks), pre-term birth (gestational age < 37 weeks) was associated with a 0.17-fold decrease in neonatal antibody titers (95% CI: 0.07-0.26; P=0.003). We observed a rapid decline of maternally acquired antibodies in neonates after birth, with a 34.3 days half-life (95% CI: 33.9-34.8). Neonatal antibody titers declined to their lowest point (3.16, 95% CI: 3.08 - 3.24) at a mean age of 10 months; this was followed by a gradual increase due to natural infection, rebounding to levels comparable to those at birth by 5-8 years of age (GMT: 4.91, 95% CI: 4.86-4.95). These findings emphasize the urgent need for targeted immunization strategies to bridge the RSV vulnerability gap during early childhood.