From colonization to causation: the links between Group B Streptococcus colonization, invasive disease, and preterm birth.

Advances in the development of vaccines against Group B Streptococcus (GBS) may soon offer an alternative approach to preventing neonatal GBS infections. However, children who are born preterm may not receive full protection from such vaccines, either because they are born before vaccine delivery, or because of less effective maternal antibodies transfer. With this as our motivation, we discuss causal links between maternal GBS colonisation, invasive GBS disease, and preterm birth, together with supporting evidence. Whilst multiple studies indicate that maternal colonisation can cause both disease and preterm birth, we argue that direct causal links between preterm birth and invasive disease require further clarification, in particular with regard to in utero onset versus postnatal early onset disease. Further, recent findings on the role of prematurity as effect modifier of GBS-related outcomes are examined. We discuss implications of these causal links and questions that should be addressed to guide studies and policy.