Causes of vertical transmission of hepatitis B virus under the at-risk prevention strategy in Japan

Authors


Correspondence Yoshinori Ito, Department of Pediatrics, Nagoya University Graduate School of Medicine, 65 Tsurumai-cho, Showa-ku, Nagoya 466-8550, Japan. Tel: 81 52 744 2294; fax: 81 52 744 2974; email: yoshi-i@med.nagoya-u.ac.jp

Abstract

The number of hepatitis B virus (HBV) carrier babies has decreased markedly since the introduction in Japan of an “at-risk” strategy for preventing HBV infection. However, elimination of HBV infection from our country appears difficult. To clarify the limitations of the at-risk strategy for preventing vertical transmission of HBV, causes of vertical transmission in a single hospital were retrospectively analyzed. The following causes were presumed in 17 carrier pediatric cases: five patients had prenatal HBV infection, HBV infection during/after the immunization program was confirmed in five cases, two patients had prenatal infection or infection during the immunization program and three cases were caused by human error (by the patients' guardians). Because their mothers were HBV-negative at screening and only developed acute hepatitis B in the perinatal period, another two cases (Cases 3 and 10) did not undergo immunization because they were not subjects of the at-risk strategy. Sequence analyses in ten patients revealed genotype C (subgenotype, C2/Ce) in nine cases and genotype A (subgenotype, A2/Ae) in one case (Case 3). In Japan, HBV subgenotype Ae has recently been found more frequently among sexually active men with acute hepatitis. There are concerns that horizontal transmission of HBV from these men to their pregnant partners could increase. These data suggest clear limitations to the at-risk strategy in Japan and the possibility that the increase in genotype A may influence vertical transmission of HBV.

List of Abbreviations
HBsAg

hepatitis B surface antigen

HBV

hepatitis B virus

Hepatitis B is a major global health problem. More than 240 million people worldwide are estimated to have chronic hepatitis B virus (HBV) liver infection and 600,000 people die each year of the acute and chronic consequences of this infection [1]. Based on the viral genome [2], HBV is currently classified into 10 genotypes from A to J, some genotypes being further classified into subgenotypes. Prevalences and genotypes of hepatitis B vary between countries. In Japan, genotypes B and C predominate among patients with chronic hepatitis B and it is generally thought that adult-acquired HBV infection rarely causes chronic hepatitis. Several reports have shown that subgenotype Ae frequently establishes chronic infection in adults [3-5]. Of note, among patients with acute hepatitis B, the frequency of subgenotype Ae appears to be increasing and spreading rapidly from the capital region all over Japan [6]. Moreover, the number of acute hepatitis B patients in Japan has not been decreasing recently [7]. Such changes may increase the frequency of HBV infection among sexually active men with acute hepatitis, and horizontal transmission of HBV from them to pregnant partners, resulting in vertical transmission of HBV.

Since the 1980s, HBV prevention programs have been implemented worldwide. Most countries have adopted “universal” vaccination strategies. However, some countries, including Japan, have adopted an “at-risk” strategy focusing on vaccination of individuals at high risk of infection. The at-risk immunization program in Japan has focused exclusively on vertical transmission and has been successful in decreasing the incidence of HBV carrier babies. Shiraki estimated that the number of HBV carrier babies decreased from 260/100,000 live births in 1986 to 24/100,000 in 1995 [8], and a recent study estimated an incidence of 5.2/100,000 live births from 2006 to 2008 (Y Torii unpublished data, 2010). However, an at-risk strategy is considered inadequate for eliminating HBV infection [9]. Moreover, as noted above, the epidemiology of HBV infection has been changing recently. To clarify the limitations of the Japanese at-risk strategy for preventing vertical transmission, we analyzed causes of vertical transmission of HBV under this strategy.

MATERIALS AND METHODS

Patients

Seventeen patients diagnosed with HBV infection by vertical transmission were followed in Nagoya University Hospital from 1998 through 2010. All patients were born after 1986, when the immunization program for babies of HBV carrier mothers was started. These patients comprised 9 boys and 8 girls, with a mean age at the time of this investigation of 9.5 ± 5.0 years (range, 3–21 years). All patients were born in Japan. According to the Japanese prevention protocol, hepatitis B hyper-immunoglobulin is administered at birth and at 2 months of age, when vaccination is started. HBV vaccine is administered at 2, 3 and 5 months of age. In accordance with national guidelines, prophylaxis is terminated in the babies whose blood tests become positive for HBsAg.

The study design and purpose were approved by the institutional review board of Nagoya University and were fully explained to all patients and/or guardians, from whom informed consent was obtained prior to enrolment.

Phylogenetic analysis

In 10 of the 17 patients, phylogenetic analyses to classify HBV subgenotypes were performed as described previously [10]. In brief, HBV DNA was isolated from peripheral blood and direct sequencing of the preS and polymerase regions performed. The neighbor-joining method [11] was used to construct phylogenetic trees.

RESULTS

Case details are shown in Table 1. Of the 17 patients, testing of cord blood showed that five had prenatal HBV infection at birth. In a further five cases, HBV infection during or after completion of the immunization program was confirmed (“failure” cases). Because HBsAg was not checked for at birth in two cases, these were classified as “prenatal infection/failure.” Three children did not receive the appropriate immunization because of human error by their guardians (“incomplete” cases). The remaining two cases (Cases 3 and 10) did not receive immunization because they were not identified as subjects of the vaccination strategy.

Table 1. Characteristics of the 17 HBV carrier cases
CaseSexAge (yr)Infection routeMother's originMaternal serotypes HBsAg/HBeAgAge when HBV infection confirmed (year)ImmunoprophylaxisImmunoprophylaxis protocolGenotype
  1. HBIg, hepatitis B hyper-immunoglobulin; NT, not tested; Vac, vaccination.
1F3Mother-to-infantJapan+/+At birth (2006)NonePrenatal infectionCe
2M3Mother-to-infantKorea+/+9 mo (2007)HBIg (2)/Vac (3)FailureCe
3F4Mother-to-infantJapan−/−2 mo (2005)NoneNon-applicableAe
4M4Mother-to-infantJapan+/+1 mo (2005)HBIg (1)Prenatal infection/failure(NT)
5M5Mother-to-infantJapan+/+6 mo (2004)HBIg (1)/Vac (3)FailureCe
6M6Mother-to-infantJapan+/+At birth (2003)HBIg (1)Prenatal infection(NT)
7F8Mother-to-infantJapan+/+8 mo (2002)HBIg (1)IncompleteCe
8F9Mother-to-infantKorea+/+1 mo (2000)HBIg (1)Prenatal infection/failureCe
9M10Mother-to-infantJapan+/+At birth (1999)NonePrenatal infectionCe
10M10Mother-to-infantJapan−/−6 mo (1999)NoneNon-applicable(NT)
11F10Mother-to-infantBrazilUnknown9 mo (2000)HBIg (1)Incomplete(NT)
12F12Mother-to-infantJapan+/+At birth (1997)NonePrenatal infectionCe
13M12Mother-to-infantJapan+/+6 mo (1998)HBIg (2)/Vac (3)Failure(NT)
14M12Mother-to-infantJapan+/+At birth (1997)NonePrenatal infection(NT)
15F16Mother-to-infantJapan+/−3 mo (1993)Vac (1)Incomplete(NT)
16F17Mother-to-infantJapan+/+2 mo (1992)HBIg (1)FailureCe
17M21Mother-to-infantJapan+/+6 mo (1989)HBIg (2)/Vac (3)FailureCe

One patient's mother's HBV serotype was not known, 14 had HBsAg-positive mothers, and two had HBsAg-negative mothers. In these two cases (Cases 3 and 10), the mothers were HBV-negative at the time of the first trimester screening, but developed acute hepatitis soon after giving birth. Case 3, a girl born in 2005, was 2 months old when acute hepatitis B was confirmed in her mother. She had no clinical symptoms and physical findings were normal. However, laboratory tests showed increased concentrations of serum transaminases. Results of serological analysis were positive for HBsAg and hepatitis B e antigen and negative for hepatitis B surface antibody. To identify the source of infection, the patient and her parents were examined and it was found that her father had been hospitalized with acute hepatitis B during the third trimester of his wife's pregnancy. Thus, it was suspected that the mother had been infected by the father during pregnancy, with subsequent vertical transmission of HBV.

Phylogenetic analysis in 10 of the 17 cases showed genotype C (subgenotype, C2/Ce) in 9 cases, whereas Case 3 had genotype A (subgenotype, A2/Ae). Phylogenetic analysis was not performed on specimens from the mothers.

DISCUSSION

In this study, one patient infected by vertical transmission had subgenotype Ae HBV. There is only one previous case report from Japan of a HBV carrier child with subgenotype Ae. In that case, father-to-child infection was confirmed by molecular analysis [12]. Kobayashi reported that the frequency of genotype A has gradually increased in patients with chronic HBV infection since 2001 and that this is accounted for by an increase in acute infection with this genotype in Japan [13]. Because HBV infection with subgenotype Ae frequently establishes chronic infection in adults, further studies are needed to clarify whether changes in the prevalence of HBV genotypes are affecting the epidemiology of vertical transmission in Japan.

It is over 20 years since the HBV immunization strategy was introduced in Japan. The protective efficacy of this at-risk strategy is 93.5% [14]. Although the incidence of vertical transmission of HBV has been decreasing, HBV is far from eradicated [7]. In this study, we considered 12 of the 17 cases were unavoidable even with administration of hepatitis B hyper-immunoglobulin and HBV vaccine, namely the five cases of intrauterine infection, five cases diagnosed during or after the immunization program and two cases of intrauterine infection or infection during/after the immunization program. Maternal viral load [15, 16] and escape mutants [16-18] are reportedly risk factors for vaccine failure and vertical transmission of HBV. However, we did not analyze these factors in the present study. Another possibility is that, in the “failure” cases, horizontal infection with HBV occurred after attenuation of the prevention conferred by hepatitis B hyper-immunoglobulin and before vaccination took effect. Three of the 17 cases did not undergo immunization because of human error by their guardians. With the at-risk strategy, it is difficult to avoid these problems. In addition, in accordance with the Japanese prevention protocol, prophylaxis was terminated when patients became HBs antigen positive. Roushan et al. showed that children who receive hepatitis B hyper-immunoglobulin and HB vaccine achieve earlier seroconversion than who do not [19]. It may be of benefit to complete prophylaxis after confirmation of HBV infection. Komatsu et al. reported on sources of transmission among children with HBV infection under the at-risk strategy [20]. Of 57 patients, 37 (65%) were born to HBV carrier mothers, whereas 20 (35%) were born to HBsAg-negative mothers. Among the latter patients, 14 had HBV carrier fathers, 2 HBV carrier siblings and 4 no family members with HBV infection. In our patients, no patient was confirmed to have been infected by horizontal transmission. Another problem is vertical transmission of HBV by seronegative mothers. We encountered two babies whose mothers who developed acute HBV infection shortly before delivery, after screening had been performed. Although this type of transmission has been reported [21], it is not a well-recognized cause of failures in the at-risk prevention strategy. Because the number of cases of acute hepatitis B is increasing in sexually active generations [6], there is concern about increased numbers of such cases under the at-risk strategy. The limitations of the at-risk strategy have been argued and some have concluded that universal vaccination is the only path to eliminating HBV [9]. Although this study assessed a small number of patients in a single hospital, it does clearly illustrate the limitations of the at-risk strategy, including difficulties in identifying all at-risk individuals. Because HBV infection of pregnant women and their partners would be prevented, universal vaccination may prevent this type of vertical transmission. Vaccination of everybody, including infants and their family members, would probably also prevent horizontal HBV transmission to infants. Moreover, it is important to educate pregnant women and their partners that HBV infection is a sexually transmitted disease even during pregnancy.

DISCLOSURE

The authors have no commercial or other associations that might pose a conflict of interest.

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