Background

Group B streptococcus (GBS or Streptococcus agalactiae) is a Gram-positive pathogen belonging to Lancefield group B. It is a common commensal of the gastrointestinal tract and colonizes 10–30% of pregnant women at vaginal or vaginal/rectal sites [1]. In pregnant women, GBS is a frequent causative agent in urinary tract infections, chorioamnionitis and postpartum endometritis, and it is also associated with poor pregnancy outcomes, including spontaneous abortion, stillbirth and preterm birth [2].

GBS is a leading cause of sepsis, pneumonia and meningitis in infants, with long-term neurodevelopmental sequelae. Neonatal GBS infections are usually divided into Early-Onset Disease (EOD, presenting at 0 to 6 days of life) and Late-Onset Disease (LOD, presenting at 7 to 90 days of life) [3]. EOD is prevented through intrapartum antibiotic prophylaxis (IAP) in women with GBS colonization or obstetrical risk factors for GBS vertical transmission.

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The term “PREPARE” designates an international project entitled “Prevention of invasive Group B Streptococcus disease in young infants: a pathway for the evaluation & licensure of an investigational maternal GBS vaccine”. PREPARE is aimed at developing a vaccine against neonatal GBS infections and is promoted by the St George University of London (UK) (see https://gbsprepare.org).

This project is part of the EDCTP2 program (European & Developing Countries Clinical Trials Partnership) that funds research for prevention and treatment of poverty-related infectious diseases in sub-Saharan Africa and it is aligned with the WHO roadmap [1]. Moreover, PREPARE is supported by Horizon 2020 (European Union’s Framework Program for Research and Innovation).

Strategies implementing IAP, especially those that screen women in late pregnancy for vagino-rectal GBS colonization (regardless of presenting risk factors) has led to a dramatic decline in the incidence rates of EOD (i.e., from 0.37 to 0.22 per 1000 live births from 2006 to 2017 in USA) [4]. However, IAP coverage is incomplete even in the best of settings. Furthermore, the burden of invasive GBS disease may be high in resource poor countries such as Africa (estimated incidence of 1.12/1000 live births) where IAP implementation is challenging [5]. Concerns have arisen as to the possible negative impact of large-scale prevention, as IAP may promote the emergence of antibiotic resistance, and early exposure to antibiotics can disrupt the development of the intestinal microbiome, with consequences in adulthood [6]. Finally, IAP has no impact on LOD, stillbirths and prematurity due to GBS, as well as a limited impact on disease in pregnant women [7]. Further strategies are urgently required to decrease GBS-associated morbidity and mortality.

There are ten known GBS serotypes (Ia, Ib and II-IX), but serotype Ia, III and V are more commonly responsible of invasive GBS disease in infants under 90 days of life. Multivalent vaccines administered to pregnant women to protect their infants against GBS disease could overcome many of the outstanding issues related to IAP and could be an effective strategy for resource-poor countries. Indeed, compared to WHO European region, in WHO African region mortality rates are 7 times higher (7/1000 vs 51/1000 LBs) [8].

Therefore, the prevention of neonatal infections through maternal immunoprophylaxis is a topic that has recently aroused wide attention. The purpose of this strategy is to induce maternal protective immunity resulting in a specific transplacental IgG passage. Indeed, recent data have shown that vaccinating pregnant women does not increase adverse events or fetal risks [9]. WHO data from developing countries show a 92% decline (from the 1980s to 2000) in neonatal tetanus case fatalities following maternal vaccination with tetanus toxoid [10].

It is estimated that to detect a 75% reduction in EOD and LOD in countries with a disease incidence of more than 1/1000 births it would be necessary to enroll about 60,000 pregnant women to study the effectiveness of the vaccine, assuming that this protects from 90% of circulating serotypes [11]. Therefore, in order to facilitate the licensure of a vaccine, the study of protective serocorrelates, followed by a demonstration of a post-license efficacy, aroused interest. Although previous studies have shown an association between serotype-specific maternal IgG titers and reduction of neonatal disease risk, no study has been able to establish with certainty a protective antibody threshold value, due to different assays for determining antibody titers or inability to compare and pool the results of different studies [12]. Vaccines have been tested against serotype-specific capsular polysaccharide and against surface proteins that are expressed in different serotypes and could then protect against specific serotypes [7, 12].

PREPARE is a non-profit, multi-center, interventional and experimental study. It aims to develop a maternal vaccine platform in Uganda, determine pregnancy outcomes and to define the extent of GBS infections in children and mothers in a sub-Saharan context. It also aims to estimate the protective serocorrelates against the main GBS serotypes that cause diseases in Europe and Africa and to conduct two trials on candidate GBS vaccines. The PREPARE project involves 7 countries across the world (Malawi, Uganda, South Africa, the United Kingdom, the Netherlands, Italy and France) and aims to develop a serum biobank, in order to define serocorrelates of protection against GBS, by using standardized antibody assays and a bacterial strains bank to study the characteristics of neonatal and maternal strains.

The overarching objectives will be achieved through 6 work packages (WPs), each with specific aims (Table 1). Italy (that belongs to WP3) is represented by a network made up of 41 centers across the country (Table 2), coordinated by the Azienda Ospedaliero-Universitaria Policlinico (Modena). The Italian network will collect at least 50 neonatal GBS invasive cases (defined as an infant with isolation of GBS from blood culture or from culture of cerebro-spinal fluid) within 2 years. Strains will be sent to the national referring center (Istituto Superiore di Sanità) for GBS typing.

Table 1 Work-Packages (WP) of the PREPARE Study: role, goals and participating countries
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Table 2 Partner of the PREPARE Consortium
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The PREPARE Italian network will collect: i) isolates from infants with invasive disease (cases), together with maternal and neonatal sera collected at the time of diagnosis of infant disease; ii) cord sera and GBS strains (of the same serotype as cases) from colonized mothers whose infants do not develop GBS infection (controls). Biological materials will be used for i) determining the concentration of specific IgG anti-GBS (serotype III the most frequent cause of neonatal disease) in the cord serum of healthy controls and in the serum of infants (aged 0 to 90 days of life) with GBS infection ii) assessing a correlation between antibody concentration and GBS disease risk and iii) validating estimates of protective serocorrelates.

Conclusions

Despite the progress made in high-income countries in the prevention of EOD, GBS remains an important cause of morbidity and mortality in the first months of life worldwide.

A maternal GBS vaccine could reduce the burden of both EOD and LOD, maternal puerperal sepsis, stillbirth and preterm delivery. A vaccine could help to overcome the inherent limitations of IAP, and could reduce unnecessary antibiotics, as well as costs and long-term disabilities consequent to GBS infection. Finally, a vaccine could be an effective strategy for resource-poor countries, where the antenatal screening and large-scale IAP might be unfeasible. PREPARE aims to undertake clinical trials of a maternal GBS vaccine, to determine pregnancy outcomes, and to estimate the protection serocorrelates against the main GBS serotypes that cause diseases in Europe and Africa.