Background
Viral hepatitis remains one of the major global public health concerns and combating hepatitis has been stated in the Sustainable Development Goals (SDG 3.3). In 2016, the World Health Organization (WHO) called for the elimination of viral hepatitis as a public health threat by 2030. In 2019, the WHO estimated that 296 million individuals were chronically infected with hepatitis B virus (HBV) and about 820 000 people died from HBV infection [
1]. HBV is most commonly transmitted from mother to child at birth (vertical transmission) as well as through horizontal transmission at the early age. Chronic infection is more likely in infants infected before the age of five [
2]. Therefore, preventive measures against HBV infection at a young age would be more efficient.
HBV infection is highly endemic in most developing countries, including Cambodia. Aside from that, Cambodia has made significant progress in combating the virus. In 2005, the national immunization program (NIP) under Ministry of Health (MOH) of Cambodia included the hepatitis B (HepB) vaccine at birth and three follow-up doses (pentavalent vaccine including HepB at 6 weeks, 10 weeks, and 14 weeks of age). Since then, the vaccine coverage has been escalating yearly from 25% in 2007 to 90.2% in 2020 for HepB vaccine birth dose and from 80% in 2006 to 100% in 2020 for three pentavalent vaccine [
3]. According to the 2017 nationwide study what we (Hiroshima University) had conducted in collaboration with NIP (Cambodia), WHO (Western Pacific Regional Office, WPRO), WHO (Cambodia), US CDC and University of Health Sciences (UHS, Cambodia), the hepatitis B surface antigen (HBsAg) prevalence was 0.56% in 5–7 years old children and 4.39% in their mothers (19–61 years old) [
4]. As a result, the WHO WPRO recognized the achievement of the regional 2017 target of HBsAg prevalence < 1% in 5 years old children in Cambodia [
5]. However, high prevalence in women of childbearing age showed the potential of vertical transmission, threatening the WHO target of eliminating hepatitis virus infection by 2030.
Following the abovementioned results from 2017 nationwide study, Cambodia needs to assess whether mother-to-child transmission of HBV still occurs or not. Therefore, this study aimed to investigate the prevalence of HBsAg among pregnant women and to assess the mother-to-child transmission (MTCT) of HBV in real-life practice in Siem Reap Province of Cambodia.
Discussion
This prospective longitudinal study was conducted among pregnant women and their newborn babies in Siem Reap, Cambodia. We determined the prevalence of HBsAg among pregnant women and evaluated the MTCT of the virus in the real-life practice of the current Cambodian health care system. In our study, HBsAg was detected in 4.28% (95% CI: 3.27%-5.28%) of pregnant women, which is consistent with the previous findings [
4]. But it is lower than the recent report from Segeral et al. (6.2%) [
14]. This HBsAg-positive rate indicates intermediate endemicity of HBV infection among pregnant women in Cambodia. Nevertheless, our result is lower than those of neighboring countries where HBsAg prevalence among pregnant women were reported as 10.8%-12.6% in Vietnam, 5.44%-8.0% in Lao PDR, and 6.2%-8.3% in Thailand [
15‐
21].
Both horizontal and vertical transmission routes exist in countries with intermediate and high endemicity of HBV, but vertical transmission is the most common [
22]. Therefore, our main concern was how much MTCT was occurring in Cambodia to evaluate the potential threat of MTCT, a significant risk factor for chronic HBV infection [
23].
The WHO recommends all countries to include the HepB vaccine in their national immunization programs. As a result, the prevalence of HBV infection has decreased globally in many countries. In Cambodia, the most recent report (as of 2020) about HepB vaccine coverage was 78.4%-90.2% for timely birth dose (within 24 h) and 88.7%-100% for the three-dose pentavalent vaccine [
3,
4]. In our study, 100% of the infants received a timely birth dose, and 98.3% received the three-dose pentavalent vaccine. Our higher HepB vaccine coverage could be due to a study effect, as pregnant women who participated were given information about HBV and were prompted to vaccinate their children. Although a cornerstone of HBV prevention, vaccination alone might not be enough to achieve the WHO target of 0.1% HBsAg prevalence in children by 2030 [
24,
25]. Moreover, the HepB vaccine may not be effective in preventing intrauterine infection, which occurs when the virus is transmitted from the mother to the fetus during pregnancy [
26,
27].
HBV intrauterine infection is defined by the presence of HBsAg and/or HBV DNA in the umbilical cord blood and has been reported to be between 5.36% and 40% [
28‐
31]. In our study, six of 17 cord blood samples collected at delivery tested positive for HBsAg. Five infants could be assessed at 6 months post-partum for HBV infection, and one of them was still positive for both HBsAg and anti-HBc, with a high viral load (1.8 × 10
9 copies/mL), despite receiving HepB vaccine and HBIG at birth. The homology analysis of HBV full genome extracted from this mother–child pair was 100%, suggesting that the virions of this infant were more likely derived from her mother during pregnancy. We did not find mutations in the “a” determinant region of the HBV surface gene, which is reportedly associated with vaccine or diagnostic escapes. This finding supports that intrauterine infection can be a cause of immunoprophylaxis failure [
32,
33].
The risk of intrauterine infections can be reduced if the mother receives antiviral therapy during pregnancy [
24,
34], prompting the WHO to recommend the administration of tenofovir in the third trimester of pregnancy until at least birth in pregnant women with high viral load, i.e.load 200,000 IU/mL (equivalent to 10
6 copies/mL) [
35]. In resource-limited countries, WHO recommends using HBeAg as an alternative to viral quantification. In our study, HBeAg-positive women had a significantly higher viral load than the HBeAg-negative women, which is coherent with other studies [
36‐
38], and supports using HBeAg as an alternative for antiviral treatment eligibility. However, HBeAg-negative pregnant women may have a high viral load in some cases. Mutations in the BPC or PC have been shown to reduce the expression of HBeAg [
39,
40], and these pregnant women may have high viral loads, thus a high risk of transmission. In our study, only one HBeAg-negative pregnant woman had a high HBV viral load, above 10
6 copies/mL. Full sequence analysis found mutations in the BCP region, consistent with a lack of HBeAg expression. This finding suggests caution in using HBeAg positivity as an eligible criterion for antiviral prophylaxis, especially in areas where HBV viral load measurement is limited. This finding also calls for further molecular studies to assess the burden of these mutations in HBsAg-positive and HBeAg-negative pregnant women to better guide MTCT prevention strategies in Cambodia.
In our study, the infant infected with HBV received a timely HepB vaccine birth dose, HBIG, and three follow-up pentavalent vaccine doses. Several studies have also reported that a residual risk of MTCT at 3%-7% despite timely vaccination [
25,
41‐
43]. The mother had a high viral load (≥ 10
6 copies/mL) and was HBeAg positive but did not receive antiviral prophylaxis. Treatment with tenofovir during the third trimester of pregnancy could have prevented the transmission of HBV from the mother to her infant. At the time of the study, antiviral prophylaxis was not recommended in Cambodia. In 2021, Cambodia established guidelines for preventing MTCT of HBV, which were adopted from the WHO 2020 recommendations. But as of 2022, these guidelines were not implemented nationwide, only in the urban capital city. Hence, vaccination at birth followed by three booster doses have been the only interventions used to reduce the MTCT of HBV, as HBIG is optional if available and affordable. Therefore, we urge nationwide implementation of the recently updated guidelines, adding antiviral prophylaxis to the current prevention measures, to achieve the WHO target of 0.01% HBsAg prevalence in children by 2030 in Cambodia. Immediate nationwide implementation of the guidelines can be challenging for Cambodia because of limited funding, low awareness, stigma, discrimination, and limited access to health care services. Therefore, sustained effort and collaboration from all the stakeholders are required to reach this important target.
This study has limitations. There was a high drop-out rate in the follow-up study, and the infection status of these infants is unknown. Therefore, over or under-estimation of MTCT in our study cannot be ruled out. However, the causes of drop-out were mainly due to restrictions during the COVID-19 pandemic making it less likely to affect the outcome of the study. Moreover, the background characteristics of those who completed follow-up and those who did not were not significantly different (
Supplementary Table). To the best of our knowledge, this is the first observational study on HBV MTCT in the real-life practice of the current Cambodian healthcare system, and the data provided are valuable for health policy making.
Acknowledgements
Our thanks will go to all women and their babies for their active participation in this study. We express our thanks to Ms. Ma Vanna and midwives from Siem Reap Provincial Hospital, Angkor Chum District Referral Hospital, and Mondule I Health Center, Mrs. Yos Socheata from National Maternal and Child Health Center (NMCHC), and Dr. Mirzaev Ulugbek from department of Epidemiology, Infectious Disease Control and Prevention, Hiroshima University for their tirelessly support during the study implementation. We also thank Mrs. Seto Kaori and Mrs. Chitomi Aratani, laboratory technicians from department of Epidemiology, Infectious Disease Control and Prevention, Hiroshima University, for their kind support during laboratory measurement.
Part of this study has been presented at the conferences: 1) The 31th Conference of Asia Pacific Association for the Study of Liver (APASL 2022), 2) The International Liver Congress 2022-the European Association for the Study of the Liver (EASL 2022), and 3) The Liver Meeting 2022 of the American Association for the Study of Liver Diseases (AASLD 2022).
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