Is health, growth and development impaired in children who are Hepatitis B-exposed but uninfected?

Abstract

An estimated 254 million people are living with chronic hepatitis B virus (HBV) infection worldwide. Many infants are born to mothers with HBV but do not themselves acquire the infection. It is unclear whether this exposure to HBV in early life - without the development of active infection - may be associated with adverse outcomes. We propose the term “HBV-exposed uninfected (HBEU)”, drawing parallels with the HIV field which recognises that children who are HIV-exposed but uninfected face an increased risk of adverse health outcomes. This paper explores the potential health consequences for children HBEU. We summarise existing evidence reporting on children HBEU, and also review existing knowledge from the HIV field that could inform insights. We hypothesise that children HBEU may be at increased risk of preterm birth, and/or impaired growth and neurodevelopmental delay, but comprehensive, longitudinal studies are currently lacking to support this. We propose a conceptual framework to hypothesise how exposure to HBV could potentially lead to adverse growth and neurodevelopment through both HBV-specific and universal pathways, and review the available evidence and research gaps. Data are needed to establish whether short- and long-term sequelae exist for children HBEU, and to inform evidence-based interventions to mitigate against detrimental outcomes. Establishing a comprehensive understanding of the long-term trajectory of health and well-being among children HBEU throughout childhood into adolescence will require longitudinal observational studies with appropriate control groups to characterise outcomes, identify risk factors and explore underlying mechanistic pathways.

1. Background

Worldwide, an estimated 254 million people are living with chronic hepatitis B virus (HBV) infection (PLWHB), and an estimated 4.5 million women living with HBV infection give birth each year [1–3]. In this article, we focus on the population of infants who are exposed to HBV in utero or peri-partum but who do not acquire the infection. For the first time, we propose the term “HBV-exposed uninfected (HBEU)” to describe this group which has not previously been formally defined. We build on concepts from the HIV field, in which “children who are HIV-exposed but uninfected (CHEU)” are at increased risk of impaired growth and neurodevelopment [4,5], and higher morbidity and mortality [6–8] compared to HIV-unexposed children. These differences in growth and development can be attributed both to HIV-specific pathways (which involve biological influence of the virus, immune activation and antiretroviral drugs) and universal pathways (through amplification or enrichment of common risk factors for poor growth and development) [5].

HBV may likewise have adverse impacts on child growth and development through biological and universal pathways. Longstanding neglect of clinical and public health infrastructure to enhance prevention, diagnosis, risk-stratification and treatment of HBV has contributed to the vulnerability of high-endemicity populations in resource-limited settings [9]. HBV prevalence is highest in populations in WHO African, South East Asian and Western Pacific regions, but has been neglected as a public health threat, with specific evidence-gaps for Africa (which now represents the region of highest new incidence due to vertical transmission) [1,9,10]. In lower prevalence settings, a significant proportion of HBV in women of reproductive age may therefore be accounted for by migrant populations [11].

Whilst strengthening the public health response to tackling paediatric HBV – now combined with HIV and syphilis in a triple elimination agenda [12] – there is an opportunity to simultaneously address the needs of children HBEU. While some published data suggest health consequences of early-life exposure to HBV (Table 1), long-term consequences for children HBEU have not been studied. Here, we propose and explore the hypothesis that there may be health consequences for children HBEU, to stimulate dialogue in clinical, research and public health communities, and to propose a framework for further research.

Table 1. Selected studies identified by a narrative review providing evidence for outcomes in infants and children HBEU. For areas where meta-analyses were available, only the meta-analyses have been reported here and other research types were excluded (e.g., pregnancy and birth outcomes). For areas where meta-analyses were not available, we present all primary literature (e.g., infant growth, vaccine response and neonatal immunity).

Reference	Study type	Setting	Findings (red = increased risk)
Pregnancy
Afraie 2023 [33]	Meta-analysis	China, Sweden, Hong Kong, USA, Germany, Thailand, Myanmar- Thailand border	Pre-eclampsia: RR: 1.10 (95% CI 1.04, 1.16) GDM: 1.16 (1.13, 1.18) Abortion: 0.97 (0.71, 1.33) Eclampsia: 1.48 (0.95, 2.29)
Jiang 2020 [100]	Meta-analysis	China, Sweden	Intrahepatic cholestasis of pregnancy: OR 1.68 (1.43,1.97)
Birth*
Afraie 2023 [33]	Meta-analysis	China, Sweden, Hong Kong, USA, Germany, Thailand, Myanmar- Thailand border	PTB: RR 1.17 (95% CI 1.14, 1.20) Neonatal death: RR 0.83 (0.67, 1.03)
Huang 2023 [148]	Meta-analysis	Myanmar-Thailand border, China, Sweden, USA, Korea	Congenital abnormalities: pooled cRR of 1.15 (95% CI: 0.92, 1.45)
Ma 2018 [32]	Meta-analysis	Greece, China, Sweden, USA, Hong Kong, Thailand, Israel, Singapore, Iran	HBsAg + /HBeAg- women vs uninfected: Preterm labour: pooled OR 1.28; (95% CI, 1.02, 1.59) PTB: pooled OR 1.16 (1.04, 1.29) HBsAg + /HBeAg+ women vs uninfected: PTB: pooled RR 1.21; 95% CI 1.10, 1.32; P  <  0.001
Cui 2017 [31]	Meta-analysis	Hong Kong, Greece, Israel, USA, Germany, Singapore, Thailand, China	PTB: pooled RR 1.26 (95% CI 1.19, 1.33)
Infant growth
Zhang 2017 [39]	Cohort study	China	Head circumference at 18 months: 3mm greater among children in non-HBV exposed group (47.3 ± 1.3) cm than children HBEU (47.0 ± 2.0) cm (P = 0.038). No difference in height, weight, increases in height/weight/head circumference each month, weight/height/head circumference for age Z scores, proportion of growth retardation and low weight.
HBV vaccine response
Jiang 2023 [46]	Cohort study	China	2163 infants HBEU, with/without passively acquired HBeAg. Neonatal HBeAg positivity correlated with transiently lower anti-HBs at 7-12m of age, no difference by 24m of age.
Huang 2019 [47]	Cohort study	China	265 infants born to mothers HBsAg positive, HBeAg pos/neg. Cord blood HBeAg status did not correlate with lower anti-HBs at 7-12m of age.
Wang 2017 [41]	Cohort study	China	328 mother-infant pairs, mother living with HBV. In 318 infants HBEU, non-response to HBV vaccine (anti-HBs < 10mIU/ml) was associated with transient post-natal HBsAg positivity (all infants tested HBsAg negative by 7 months)
Koumbi 2010 [48]	Cohort study	Greece	46 mother-children HBEU infant pairs, mothers HBsAg + /eAg- vs. HBsAg neg. All neonates responded to vaccination and HBEU. 50% had viral antigen induced IFNg suggesting prior exposure to viral antigens, however this did not impair T cell response to vaccination.
Badur 1994 [42]	Cohort study	Turkey	14 infants HBEU. 22% did not produce anti-HBs 1 month after 3rd dose of HBV vaccine, all of whom had detectable HBV DNA in fetal PBMC at birth.
Neonatal immunity
Hong 2015 [49]	Cohort study	Singapore	HBV exposure in utero associated with a state of trained immunity, characterised by innate immune cell maturation and Th1 development. NB. HBV infection status not reported.
Shrivastava 2013 [149]	Cohort study	India	22 newborns born to mothers living with HBV . Newborns born HBsAg-ve (n = 12) maintain CD8 + T cell function (IFNg production and CD107A expression) v.s. HBsAg +ve (n = 10) newborns. NB. All subsequently given HBV-BD vaccine, infection status at 12 months not reported.

Acronyms: BD = birth dose, HBV = hepatitis B virus, HBEU = hepatitis B exposed uninfected, PBMC = Peripheral Blood Mononuclear Cells, SGA = small for gestational age, LBW = low birth weight, PTB = preterm birth, GDM = gestational diabetes mellitus, ICP = intrahepatic cholestasis of pregnancy, IUGR = intrauterine growth restriction, LGA = large for gestational age, PROM = preterm rupture of membranes, RCT = randomised controlled trial

* NB. These studies reported outcomes in mothers living with HBV, regardless of infant infection status.

2. Search strategy

The aim of the paper is to develop hypotheses about outcomes in children HBEU, assimilating a theoretical body of data which can be applied to inform the development of new evidence. As there are insufficient data for a systematic review, we adopted a narrative review approach. We undertook our search of the PubMed database between March 2023 - April 2024. We used search terms for “HBV”, “neonate/infant/child”, “growth/neuro-development” and “exposure/uninfected” using MeSH terms which were supplemented with relevant free text terms (truncated where appropriate to capture maximum resources). We narrowed the search to incorporate areas of interest, (including specific birth outcomes, microbiome, prevention of mother to child transmission (‘PMTCT’), breastfeeding). The search result was limited to articles that were written in English with a priority to include publications from the last 10 years. For areas where meta-analyses were available, we focus on reporting the meta-analysis data and other research types were excluded (e.g., pregnancy and birth outcomes). For areas where meta-analyses were not available, we present the primary literature (e.g., infant growth, vaccine response and neonatal immunity). The database searches were complemented with manual review of the reference lists of relevant articles.

3. Children who are HIV- vs. HBV-exposed uninfected

The hypotheses presented here mirror concepts in the HIV field. There are sparse data available for HBEU and many useful parallels can be drawn between HBV and HIV; however there are also key differences which influence how children HBEU may be defined, how problems may manifest, and how children HBEU should be identified (Table 2 and Fig 1). Key differences include the ongoing risk of postpartum breast milk transmission in HIV compared to HBV, testing time-points, and the evolving prevention landscape for each infection.

Table 2. Comparison of HBV and HIV transmission, testing, treatment and prophylaxis. Testing, treatment, prophylaxis and elimination targets based on WHO guidelines [12,150–152].

	HIV	HBV
Natural history (without intervention)	Percentage vertical transmission without intervention 15–45% Timing of vertical transmission [153]: - 10–20% intrauterine; - 35–45% intrapartum; - 35% postpartum (via breastfeeding).	Percentage vertical transmission without intervention: HBeAg+ 70–90%, HBeAg- 10–40% [13–16] Timing of vertical transmission: - 5–10-% intrauterine [154,155]; - 90–95% intrapartum [156]; Potential also for horizontal transmission during early childhood (1–5 years).
Infant testing	Longitudinal molecular testing required due to ongoing transmission risk via breastfeeding and passive transfer of maternal antibody, e.g., birth, 4–6 weeks, 9 months, 18 months and end of breastfeeding risk period [157].	Testing with HBsAg recommended at 12 months [17]. Passive acquisition of viral DNA and antigens, in addition to vaccine-derived HBsAg in uninfected infants complicates earlier testing.
Treatment and prophylaxis during pregnancy + /- breastfeeding	Lifelong ART for all pregnant and breastfeeding women living with HIV.	Tenofovir during pregnancy and early postpartum if HBV VL ≥ 200,000 IU/mL or HBeAg positive. If VL or HBeAg testing not available, prophylaxis can be offered to all who test HBsAg positive.
Infant prophylaxis	ART duration dependent on risk status (4–6 weeks for low risk, 12 weeks for high risk).	HBV-BD vaccination followed by accelerated primary HBV vaccination course. HBIg if VL ≥ 200,000 IU/mL or HBeAg positive.
Elimination targets [12]	Zero new infections among infants and young children and achievement of the ‘95-95-95’ targets. EMTCT impact targets: Population case rate of new paediatric HIV infections due to MTCT of <50 cases per 100,000 live births MTCT rate of HIV of <2% in non-breastfeeding populations or <5% in breastfeeding populations.	95% reduction in incidence of chronic HBV infections. EMTCT impact targets: <0.1% prevalence HBsAg in children 0–4 years old. Additional target <2% MTCT rate (for countries using targeted timely HBV-BD vaccine).
Global coverage of VT prevention programmes	84% global coverage of ART in 2023.	45% coverage globally of HBV-BD vaccination in 2023 [158]. HBIg coverage 13% globally.
Number of children estimated to be born exposed uninfected	16.1 million global population (2023 [159]), 14.6 million of whom are in WHO African region.	Not previously estimated.

Acronyms: HBV-BD = hepatitis B birth dose vaccine, HBsAg = hepatitis B surface antigen, HBeAg = hepatitis B e-antigen, VL = viral load, HBIg = Hepatitis B immunoglobulin, ART = antiretroviral therapy, MTCT = mother to child transmission, EMTCT = elimination of MTCT.

Fig 1. Comparison between infant viral markers, prophylaxis, treatment and testing strategies for prevention of vertical transmission of HIV and HBV [25,152,160].

(A) HBV exposed uninfected infants (HBEU). Transient HBsAg positivity is seen due to Transplacental transfer of HBsAg (T) and Vaccine derived HBsAg (V), highlighting the need for delayed testing and confirmation of infant HBV infection status at 12 months of age. Data adapted from a single study [25] of 139 HBEU infants who received combined HBV immunoprophylaxis with HBIG and HepB-BD vaccine followed by two further doses of HBV vaccine at 1 and 6 months of age (indicated on graph). WHO guidelines recommends universal HBV immunisation, including HepB-BD, to be given during infancy with a minimum of three doses, however a total of four doses is recommended. The timing of the fourth dose varies between country schedules, however recommendation is that each dose is separated by at least 4 weeks [25,152,160]. (B) Children who are HIV exposed but uninfected (CHEU). Transient maternal HIV-1 antibodies can take 18 months or longer to be cleared, therefore testing for chronic HIV infection in infants prior to 18 months of age using antibody tests is inaccurate. WHO advice serological antibody testing at 18 months or 3 months following cessation of breastfeeding [152]. HIV RNA levels remain undetectable in CHEU throughout the first 18 months of life. HIV antibody has been detected after 18 months of age. See WHO guideline for details of low/high risk infant ARV prophylaxis [152]. NAT - Nucleic acid test, DBS - Dried blood spot, ARV - Antiretroviral medication. Created in BioRender. Parker, E. (2024) https://BioRender.com/i19y222.

4. Perinatal HBV exposure in children

Vertical transmission (also referred to as ‘mother-to-child transmission’ (MTCT), typically occurring peri-partum but also potentially in utero) is the commonest route of HBV infection worldwide. Without intervention, the rate of vertical transmission from mothers testing HBeAg-positive is between 70–90%, and if HBeAg-negative 10–40% [13–16]. Vertical transmission can be prevented with interventions prenatally (antenatal HBsAg screening ± antiviral prophylaxis), perinatally (hepatitis B birth dose vaccine (HepB-BD) ± hepatitis B immunoglobulin, HBIg) and postnatally (3–4 follow-up vaccine doses) [17]. This vaccine schedule leads to immunological protection in ~95% of children, although variability in development of anti-HBs antibodies post-vaccination has been observed, and is lower in infants who are HIV-infected [18]. In 2000, HepB-BD coverage within 24 hours of birth was 5% globally, rising to 45% in 2023. This is set to improve further, as in 2023 Gavi (the Vaccine Alliance) recommitted to supporting the introduction of HepB-BD vaccine, in addition to hexavalent infant vaccination which delivers HBV vaccine alongside other routine childhood immunisations in the first 2–6 months after birth [19]. As HBsAg testing, antenatal tenofovir prophylaxis, and BD vaccine coverage expands, the majority of children will be protected against chronic HBV infection, and therefore among all children perinatally exposed to HBsAg, the proportion of those born HBEU will increase. The absolute number of children HBEU will also depend on the number of children being born to women with CHB, influenced by the number of women of childbearing age with CHB worldwide [20] and changes in fertility rate. The global response to HBV elimination is off track, and in many populations (despite HBV being incorporated into routine vaccine programmes since the mid-1990’s), population prevalence of HBV is still moderate or high [21].

5. Serological profiles in diagnosis of children HBEU

To define a child as HBEU, it is critical to rule out HBV infection, which is typically based on detection of the viral envelope protein, hepatitis B surface antigen (HBsAg). Excess HBsAg is secreted as non-infectious particles, which may have a role in immune modulation [22]. Hepatitis B e-antigen (HBeAg) is a viral accessory protein secreted from infected cells, is associated with high viraemia, and has an immunomodulatory function [22]. The fetus may be exposed to very high loads of both HBsAg and HBeAg in utero; these antigens can be passively transmitted transplacentally and intrapartum, and can therefore be detected in uninfected infants for up 12 months postpartum [23–25]. Furthermore, HBsAg may also be detectable in blood up to 21 days days following a dose of HBV vaccination [26,27]. It is therefore difficult to use HBsAg as a test of HBV infection in early life. Most guidelines recommend HBsAg testing at 12 months of age [17,28,29], which maximises the chance of detecting all-cause infant HBV infection during the first year of life, and reduces the risk of misinterpreting placentally-transferred or vaccine-derived HBsAg as active infection [30]. Timing of HBV exposure, infection and diagnostic tests differ from HIV (Table 2; Fig 1) with definition of HBEU ultimately depending on confirmed perinatal exposure but persistent absence of HBV infection in the infant (Table 3).

Table 3. Classification of perinatal HBV exposure and infection.

		Maternal HBsAg status during pregnancy
		Positive	Negative
Infant HBsAg status at 1 year	Positive	HBV infected infant – likely vertical transmission, also possible early life horizontal transmission	HBV infected infant - horizontal transmission
	Negative	HBV exposed uninfected (children HBEU)	Non-exposed, non-infected

6. Adverse impact of maternal HBV on birth and childhood outcomes

Several meta-analyses, including data from China, Hong Kong, Singapore, USA, Sweden, Germany, Greece, Thailand-Myanmar border, Israel and Iran, have documented an association between maternal HBV infection and increased risk of preterm birth (PTB) [31–33], which may be specifically influenced by maternal HBeAg status [32,34,35]. Due to the mixed evidence base, we only report the meta-analyses here. However, evidence is not consistent on the association between HBV exposure during pregnancy and PTB, and more data are needed to understand specific risk factors and causality. Irrespective of cause, small vulnerable newborns, who are either low birthweight (LBW), preterm (born alive before 37 weeks’ gestation) and/or small for gestational age (SGA; born with birthweight below the 10th centile for gestation) [36] are at increased risk of both short- and long-term health consequences including neonatal mortality and morbidity, delayed growth, neurodevelopmental delay, and non-communicable diseases later in life [37,38].

Minimal data on growth and neurodevelopment in children HBEU have been published. A population cohort study in China showed a smaller head circumference (47.3 + /- 1.3 cm vs. 47.0 + /- 2.0 cm, p = 0.038) but no other difference in growth parameters at 18 months of age in children born to mothers living with HBV vs. women uninfected with HBV (abstract only available) [39]. Vertical transmission of HBV at 18 months was 1.0% (1/97); child development was not assessed.

There are several caveats in interpreting current data. First, studies of birth outcomes are inconsistent in reporting infant HBV testing, therefore it is not possible to distinguish the impact of perinatal HBV exposure from infection. Second, a notable data gap exists for the WHO African region, which has a high HBV prevalence and incidence (accounting for ⅔ of new incident infections worldwide) [1], but is not represented in any meta-analyses of birth and pregnancy outcomes. Due to the diverse factors that influence outcomes, the effects of HBV exposure and infection may be different between population settings, and more complete global data are required.

7. Potential pathways for negative health outcomes in children HBEU

7.1. Specific immune responses to HBV

The impact of HBV on the neonatal immune system has largely been studied in the context of vertical transmission, with less attention given to children HBEU. The ultimate outcomes of HBV exposure and either infection or immunity are a complex interplay between natural and vaccine induced immune responses. In the case of active immunisation, the infant must mount a timely immune response to vaccine-antigen. HepB-BD vaccine plus follow-up doses prevent vertical transmission in ~95% infants, demonstrating that most HBV-exposed infants are able to mount a neutralising antibody response.

It is possible that immune response to vaccine may be altered by high exposure to viral antigens and/or DNA in utero, contributing to the ~ 5% of vaccinated infants HBEU who have anti-HBs titres <10 IU/ml after receiving ≥3 vaccine doses [40]. Transplacental HBsAg transfer has been associated with non-response to HBV vaccination [41], as has the transplacental passage of HBV DNA and presence of HBV DNA in neonatal PBMCs [42]. However, although it has been proposed that neonatal exposure to maternal HBeAg contributes to immune tolerance [43–45], exposure to maternal HBeAg in children HBEU does not appear to have a long-term impact on anti-HBs titres post-vaccination [46–48].

7.2. HBV-specific effects on immune response to other pathogens

HBV exposure might also influence the neonatal or infant immune response to other pathogens, either with beneficial or detrimental effects on morbidity and mortality. Analysis of cord blood demonstrated increased innate immune cell maturation and Th1 development in HBV-exposed compared to HBV-unexposed neonates, with increased activation of CD123⁺ plasmacytoid dendritic cells and natural killer cells, mature cord blood monocytes, low IL-10 and high IL-12p40 and IFNα2 [49]. This picture is consistent with a ‘trained immunity’ hypothesis, in which exposure to bacterial or viral infections and live vaccines can protect infants against unrelated pathogens by boosting functional innate immunity [50,51]. Such a profile in children HBEU might confer advantages upon exposure to unrelated pathogens. This is analogous to murine studies of CMV, in which exposure augments responses to other unrelated pathogens such as Listeria [52].

7.3. Maternal immune/inflammation mediated effects

Oxygen delivery, exposure to maternal hormones, immune activation/inflammation and toxins, play a critical role in shaping fetal development. ‘Maternal immune activation’ (MIA), as a result of infections and inflammation in general, can impact fetal growth and development through an increase in pro-inflammatory cytokines (e.g., IL-6 and activation of Th17 cells) which may potentially affect child growth and development, including cognitive and physical impairments that persist beyond infancy [53]. Maternal stress or infection can also lead to increases in pro-inflammatory cytokines leading to endocrine disruption (e.g., via the hypothalamic-pituitary-adrenal axis). This can lead to neurodevelopmental reprogramming via epigenetic changes such as DNA methylation of promoter regions (e.g., the glucocorticoid receptor promoter, leading to altered glucocorticoid receptor expression in the hippocampus [54]) and histone methylation (e.g., influencing differentiation of neural precursor cells) and therefore alter fetal growth and development [55,56]. HBV infection has the potential to modify these components of the maternal milieu, thereby impacting childhood outcomes. However, neither the impact of MIA nor neurodevelopmental reprogramming has been characterised in children HBEU.

Direct viral effects may also arise through placental infection and inflammation. Placental HBV infection is recognised [57], which can cause trophoblast apoptosis [58] allowing HBsAg, HBeAg and DNA to cross the placenta [41,59]. Other maternal infections during pregnancy have been shown to affect the developing fetal immune system (independently of the vertical transmission of pathogens) through sensitisation to pathogen antigens after transplacental transfer, inflammatory responses in placental syncytiotrophoblasts, and reduced transmission of maternal antibodies [60]. It is therefore feasible that placental HBV infection could impact the structure and morphology of the placenta and influence outcomes in children HBEU.

A rise in the liver enzyme alanine aminotransferase (ALT) during pregnancy may be evidence of maternal immune activation in HBV, due to a cytotoxic T-cell response against infected hepatocytes. Typically, such a ‘flare’ would be associated with reduced viraemia, and the potential for clearance of HBsAg [61]; however, a rise in ALT can also signify an increase in viraemia (e.g., reflecting transient immunosuppression related to pregnancy [61]), and/or could be mediated by other liver disease during pregnancy (e.g., obstetric cholestasis, preeclampsia and HELLP syndrome [62]). Such ALT flares are typically mild or self-limiting during pregnancy and postpartum [63] and some data suggest that ALT flares in HBV in the third trimester are not linked to adverse outcomes [64]. However, other studies have shown a link between raised ALT in pregnancy and the development of gestational diabetes or PTB [65]. Links between high ALT levels in HBV and cytokines such as IFN gamma, IL-2 and IL-17 producing CD4 + cells, suggesting potential mediation of a systemic inflammatory response [66], but the clinical significance on the fetus has not been determined.

7.4. Microbiome effects

Maternal gut microbial dysbiosis in HIV infection can be associated with adverse birth outcomes including LBW [55,67–69], and there is also evidence that HIV exposure influences the gut microbiome of HIV-uninfected infants resulting in over-maturation and over-diversification [70]. Microbial dysbiosis has also been reported in chronic HBV infection [71–77], with evidence of increased microbial translocation (e.g., evidenced by systemic LPS and sCD14), markers of enterocyte death (I-FABP) and markers of systemic inflammation [73]. On these grounds, disruption to the maternal microbiome could be a pathway to adverse fetal or infant outcomes. Influences of the maternal gut and vaginal microbiome on fetal growth and development include delivery of metabolites essential for fetal growth, and vertical transmission of the maternal microbiota peripartum [78]. It is also thought that adaptations to the maternal gut microbiome are necessary for development of the fetal gut-brain axis both pre- and post-natally, with studies citing impacts on fetal brain neurogenesis, impact on the developing fetal immune system (specifically fetal regulatory T cells), and deficiencies in short chain fatty acids impacting fetal enteric nervous system development [79]. Furthermore, it has been hypothesised that microbial dysbiosis may impact infants’ ability to clear HBV. In a mouse model it was demonstrated that 12-week-old mice with a stable microbiome cleared HBsAg within 6 weeks following injection with HBV; however, those who had not fully developed their gut microbiome by 6 weeks did not clear HBsAg by 26 weeks. This study posited an immunomodulatory role of the gut microbiome, suggesting an impact on HBV clearance [74].

Environmental enteric dysfunction (EED), a common condition in some populations (more typically in low/middle income countries), characterised by impaired gut barrier function, intestinal inflammation and microbial translocation [80], is associated with an altered gut microbiome (although the direction of cause and effect is not known). Maternal EED may increase the risk for PTB, LBW, and impaired neurodevelopment [81], although reports are conflicting [82]. Neither gut dysbiosis nor EED have been studied in the context of maternal HBV infection, impact on vertical transmission or impact on children HBEU.

7.5. Drug toxicity

Another potential influence on fetal development and outcomes is exposure to nucleos/tide analogue (NA) agents taken during pregnancy, most commonly tenofovir disoproxil fumarate (TDF). The use of TDF in the context of high viraemia during pregnancy is effective in prevention of vertical transmission [83–85], and a wide literature (from both HBV and HIV) shows that benefits exceed risks. Large studies and meta-analyses have provided reassurance by showing no significant increase in congenital malformations or infant bone mineral density (BMD) up to one year of age [85–89]. Tenofovir safety in pregnancy has also been demonstrated in trials of HIV pre-exposure prophylaxis (PrEP), in which there is no evidence of adverse pregnancy outcomes or restricted fetal growth [90,91]. Longer-term studies following fetal TDF exposure reported normal physical growth, BMD and neurodevelopment in children up to 192 weeks in China [92], and a Taiwanese study showed comparable long-term growth, renal function, and bone development up to age 6–7 years [93]. On these grounds, it is unlikely that drug exposure has a contributory influence on any adverse outcomes in children HBEU, while there are substantial benefits in prevention of long-term infection.

8. Universal risk factors for negative health outcomes in children HBEU

8.1. Maternal co-morbidity

Maternal HBV infection may increase the risk of common pregnancy-related maternal comorbidities associated with adverse neonatal outcomes [94,95], including gestational diabetes mellitus (GDM) [33,96–98], pre-eclampsia [33,99] and intrahepatic cholestasis of pregnancy (ICP) [33,100]. It has been hypothesised that HBV drives GDM via raised tumour necrosis factor alpha and ferritin, leading to increased insulin resistance, and is associated with elevated risks of PTB, low Apgar scores, macrosomia, respiratory distress syndrome, neonatal jaundice and admission to neonatal intensive care [101]. Risk of pre-eclampsia is higher with maternal HBV infection, leading to less blood flow to the fetal-placental unit and reduced fetal growth [33,102]. The risk of ICP is higher in HBV infection [100,103], hypothesised to be driven by an interaction between modified immune cell function, hormonal changes and the shared entry receptor/bile acid transporter Na+-taurocholic acid co-transporting polypeptide (NTCP) [100]. It poses risks to the fetus and has been associated with increased risk of PTB and LBW [104,105].

8.2. Psychosocial risk factors

HBV infection has been associated with socioeconomic deprivation in a wide variety of settings (e.g., published data from England, Brazil, Japan and Turkey [106–109]). This association is often associated with low advocacy and education for populations at risk, barriers to care for marginalised groups, trends in migration, and a lack of sustained programmatic investment in clinical and public health interventions. ‘Key populations’ are also at increased risk of HBV exposure and infection and negative health outcomes, (for example, including vulnerable migrants and people who inject drugs), in whom a complex interplay of stigma and discrimination, competing health and social priorities, and systematic barriers to care, can adversely affect access to interventions for HBV prevention, diagnosis and treatment [110,111]. While these factors - and the extent of their influence - clearly vary between individuals and settings, parental HBV infection may be a component of a complex milieu of influences on long-term health outcomes, with children suffering from impacts on physical and psychosocial development, health behaviours, nutrition, education, and social exposure and opportunities, irrespective of their own HBV status [112,113]. PTB and LBW are also associated with socioeconomic status [114,115]. To better understand these influences, studies of HBV infection and exposure that record and adjust for socioeconomic factors are required in different geographic and sociodemographic population settings.

HBV-related mortality in adults carries a risk of orphanhood for their children, which is a risk factor for extreme poverty. This risk has been quantified as 1.9-fold (95% CI 1.1–3.3) increased hazard of all-cause mortality and 13.3-fold (95% CI 3.9–45.5) increased hazard of liver-related mortality compared to uninfected in a US study [116], but varies significantly between populations. The impact of HBV diagnosis on adult mental health has been reported with high rates of depression and anxiety and decreased quality of life scores [117–119], which may adversely influence cognitive development, school completion and childhood mental health in their offspring [120,121]. However maternal mental health has not been well studied, with only one retrospective cohort in China suggesting no correlation of HBV with post-partum depression [122].

HBV infection is associated with stigma and discrimination [123], with diverse influences on maternal and family health, education, employment and travel opportunities, health-seeking behaviour, and access to healthcare services [124–126], both for individuals living with infection and for family members [9]. To date, there has been no consistent approach to measurement of stigma in people living with HBV, but a partnership between the World Hepatitis Alliance and the European Centre for Disease Prevention and Control (ECDC) has piloted a stigma survey in Europe (https://www.worldhepatitisalliance.org/stigma/) which will support expansion of data in this field.

A systematic review in 2022 demonstrated high rates of intimate partner violence in pregnant women living with HIV, associated with poor mental health, reduced antiretroviral adherence, and reduced access to healthcare or health-seeking behaviour [127]. Equivalent data are lacking for HBV, but it is possible that some PLWHB are likewise at higher risk of intimate partner violence with health, economic and social disadvantages for their children.

Healthcare systems in most societies exclude certain groups, for whom services are inadequately designed and out of reach. Key populations include refugees and vulnerable migrants, traveller communities, the LGBTQ+ community, sex workers, incarcerated people, and those experiencing homelessness, mental illness and substance addiction. Furthermore there are substantial health inequities by geographic regions, and in certain populations which influence the prevalence, outcomes, and access to treatment for PLWHB [128,129]. Due to complex overlapping risk factors, HBV infection is enriched in these groups, enhancing inequity through a combination of morbidity and inadequate access to healthcare.

Children born to a subgroup of individuals who have contracted HBV through injecting drug use, are at risk of the direct consequences of drug exposure in utero, and indirect consequences of poverty, maternal malnutrition, mental health problems and childhood [130,131]. HBV prevalence in people who inject drugs is highly variable by setting (estimated between 2–18%) [132], so this risk intersects with other geographical and societal factors.

The risk factors discussed here are equivalent in children who are HBV exposed and those who are HBEU, and further work is needed to determine if these groups have comparable outcomes, how they compare to matched children in the same populations who are unexposed and uninfected, and how outcomes vary by population and geography.

8.3. Reduced breastfeeding in mothers living with HBV infection

HBV can be detected in breast milk, but the virus has not been proven to be infectious, and transmission has not been confirmed in animal models [133]. Guidelines therefore recommend breastfeeding [134], as any small transmission risk should be mitigated by universal HepB-BD vaccination alongside other prophylactic interventions, and the health advantages of breastfeeding outweigh potential risks [135–137]. However, in practice, there may be concerns among both PLWHB and reluctance among healthcare professionals to advocate consistently for universal breastfeeding which could reduce breastfeeding in individuals living with HBV, potentially associated with adverse impacts on infant health [138–141].

8.4. Co-infection

HBV coinfection with other blood-borne viruses (HIV, HCV and/or HDV) may impact on maternal and infant health [142,143]. In CHEU, maternal severity of disease has been shown to increase infant morbidity, and it is possible that the acceleration of maternal liver disease caused by co-infection is a contributor to morbidity in children HBEU. Other infections, some of which are more prevalent in people living with HIV, could also contribute to adverse outcomes [5], e.g., toxoplasma, rubella, cytomegalovirus, herpes simplex virus, zika virus, and syphilis [144].

9. Geographic and temporal influences on prevalence and outcomes of children HBEU

Differences in regional HBV prevalence and coverage of initiatives to prevent vertical transmission impact the global prevalence of children HBEU. Differences in prevalence reflect the complex milieu of socioeconomic, demographic, host genetic, environmental and other factors [145,146], which also influence education and awareness, accessibility and affordability of diagnostics, perinatal care, and prophylactic interventions, and long-term clinical care for both mothers and infants. Risk factors for adverse birth outcomes are likely to differ between low prevalence vs. endemic settings. These geographically-distributed influences could also modulate the phenotype of children HBEU between populations.

The global population of children HBEU will change over time. One influence is expanding vaccination coverage, such that fewer mothers will be living with HBV infection, lowering overall peripartum exposure. However, given the large undiagnosed population reservoir of HBV, and incomplete coverage of vaccination programmes, immunisation will take many decades to deliver elimination, and there is still a priority need for focus on peripartum management of HBV. Interestingly, there is a converse influence which increases the proportion of infants who are HBEU: as better access to diagnosis and preventive interventions are implemented peripartum, among all infants born to mothers living with HBV infection, a a smaller proportion will acquire HBV infection vertically and a higher proportion will be HBEU.

10. Evidence gaps and limitations

To date, the majority of published evidence addressing the impact of early-life HBV exposure considers maternal and fetal complications during pregnancy, or at delivery, without considering potential repercussions later in infancy and childhood. As much of the global burden of HBV is undiagnosed, many children HBEU are unrecognised, and most of the studies we reviewed did not differentiate between HBEU vs exposed and infected infants. Furthermore, existing studies are heterogeneous and many do not clearly describe or account for other factors that might contribute to PTB, LBW or neurodevelopmental delays. Some adverse outcomes, such as miscarriage or neonatal mortality, preclude determining fetal or infant HBV infection status. On these grounds, we have here presented hypotheses as a narrative format rather than using formal methodology for a systematic review.

Influences and outcomes may be setting-dependent. Studies that inform a better understanding of the immunological environment in utero, the influence of maternal microbiome, and factors influencing vaccine response could provide mechanistic insights into diverse outcomes. Such studies are necessary to develop an evidence-base for intervention, but these are currently lacking. Prospective studies are needed to determine whether the immunological changes in children HBEU persist long-term and are associated with altered susceptibility to, and morbidity and mortality from, other infectious diseases.

There are no data representing children HBEU in the WHO African region for HBV mono-infected individuals, and only one study we could identify of HIV-HBV coinfected individuals [143]. This represents a large data gap in a region with high HBV prevalence, universal risk factors for poor developmental outcomes, high prevalence of exposure to HIV and other pathogens including Mycobacterium tuberculosis and malaria [147], and ubiquitous cytomegalovirus infection in infancy, which may collectively contribute to adverse outcomes. Future research should also address regional differences in the outcomes of children HBEU, and seek to assess the impact of maternal health, economic and societal factors. We outline key research gaps for children HBEU in Fig 2. Future studies will need to address the challenges of keeping uninfected children under follow-up to assess long term outcomes, with registries held on platforms for data capture, with standardised definitions and time points. As electronic healthcare data collection and links to other datasets advance (e.g., educational outcomes, mental health), there are opportunities for use of routine data to interrogate long-term outcomes. The expansion of data collection has resource implications, but feasibility and efficiency will be improved by linking to efforts to advance towards HBV elimination targets.

Fig 2. Potential contribution of HBV-specific and universal factors to adverse outcomes in utero and long term growth and development and key areas for future research in children HBEU.

Created in BioRender. Lumley, S. (2023) https://BioRender.com/m71a556.

11. Conclusion and recommendations

This review sought to understand if there are potential health and developmental impacts for children HBEU, and found that evidence is currently lacking. There is a paucity of longitudinal studies to evaluate the long term health outcomes of these children, and the small number of existing studies means that we cannot yet differentiate the health outcomes of children HBEU from infants with HBV infection.

We hypothesise that children HBEU may be at risk of adverse outcomes due to a complex interplay of both HBV-specific and universal mechanisms, with a precedent from the HIV field. Data are needed to establish whether short- and long-term sequelae exist for children HBEU, and to inform evidence-based interventions to mitigate against detrimental outcomes. Establishing a comprehensive understanding of the long-term trajectory of health and well-being among children HBEU throughout childhood into adolescence will require longitudinal observational studies with appropriate control groups to characterise outcomes, identify risk factors and explore underlying mechanistic pathways (Fig 2). Interventions during pregnancy and infant vaccination programmes are opportunities for HBV diagnosis, prevention and education, and afford opportunities to enhance maternal and child health [3]. Studies should incorporate areas of different prevalence, represent different environmental influences, explore outcomes in high vs. low/middle income countries, identify high-risk subgroups, and investigate potential mechanistic pathways. Such data are crucial to inform future prevention and intervention studies.

We need evidence to inform guidelines and implementation of public health strategies to support the wellbeing of children HBEU and to mitigate potential long-term health risks, ensuring children HBEU survive and thrive.

Funding Statement

SFL is funded by a Wellcome Doctoral Training Fellowship (grant number 220549/Z/20/Z). EP is funded by an NIHR Academic Clinical Fellowship (grant number ACF-2022-19-004). PCM receives core funding from the Francis Crick Institute (Ref: CC2223) and from University College London Hospitals NIHR Biomedical Research Centre (BRC). AJP is funded by Wellcome (grant number 108065/Z/15/Z). This research was funded in whole, or in part, by the Wellcome Trust. For the purpose of Open Access, the author has applied a CC BY public copyright licence to any Author Accepted Manuscript version arising from this submission.