Prevalence of group B Streptococcus colonization in pregnant women and serotype specific immunity among mother-newborns dyads in Ghana and Zimbabwe

BACKGROUND: Rectovaginal colonization with Streptococcus agalactiae (Group B Streptococcus; GBS) during pregnancy is a major risk factor for neonatal invasive GBS disease and adverse birth outcomes. We investigated the prevalence of maternal GBS colonization, serotype-specific immunoglobulin G (IgG) immunity in mother-newborn dyads, and GBS associated stillbirths in Ghana and Zimbabwe. METHODS: A prospective cohort of 1238 pregnant women and their infants was enrolled (2018-2020). Recto-vaginal swabs collected at ≥36 weeks gestation or prior to delivery and chest aspirates from stillbirths were cultured for GBS. Isolates were serotyped using latex agglutination. Maternal and cord blood samples were analysed for GBS serotype-specific IgG using a multiplex bead-based immunoassay. RESULTS: The prevalence of recto-vaginal GBS colonization was 15.6% (68/437) in Ghana and 15.2% (50/329) in Zimbabwe. GBS was detected in 33.3% (3/9) of stillbirths. Dominant colonizing serotypes were Ia (25%), III (32%), and V (27%). Maternal IgG concentrations were significantly higher in Ghana than in Zimbabwe for serotypes Ib (0.11 vs 0.02 μg/mL; p = 0.0001), II (0.24 vs 0.06 μg/mL; p = 0.0001), and IV (0.03 vs 0.006 μg/mL; p < 0.0001). Transplacental IgG transfer ratios ranged from 0.81 to 1.08; highest for serotype II and lowest for Ia. Newborns with IgG above the serological thresholds for risk reduction for serotype Ia and III were 25.6% (Ia) and 38.4% (III) in Ghana; and 34.1% (Ia) and 26.8% (III) in Zimbabwe. CONCLUSIONS: The predominant GBS serotypes distribution in Ghana and Zimbabwe aligns with global epidemiological patterns. Despite evidence of natural maternal IgG immunity and efficient transplacental transfer, most newborns lacked protective antibody levels against GBS. A maternal GBS vaccine targeting dominant serotypes may reduce the risk of GBS neonatal invasive disease and GBS-associated stillbirths.