Increased Prevalence of HBV Envelope Mutants in Taiwan: An Emerging Public Healt

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Increased Prevalence of HBV Envelope Mutants in Taiwan: An Emerging Public Health Risk or a False Alarm?

    Daniel Shouval

    Stephen Locarnini

Liver Unit, Hadassah-Hebrew University Hospital, Jerusalem Israel

Victorian Infectious Disease Reference Laboratory, North Melbourne, Victoria, Australia

published online 22 June 2012.

The development of plasma-derived hepatitis B vaccines in the late 1970s and recombinant yeast derived vaccines in the late 1980s has paved the road for global control of hepatitis B virus (HBV) infection. At present, 179 nations accepted the World Health Organization recommendation to start universal vaccination of newborns with a ∼75% coverage.1 The implementation of universal vaccination in newborns has led to an impressive decline in prevalence of hepatitis B surface antigen (HBsAg), chronic hepatitis B (CHB), and hepatocellular carcinoma in children and adults as reported from Taiwan, Gambia, China, and countries with a high rate of infection.2 Yet, over the past 2 decades, sporadic reports have documented the emergence of escape mutants to the HBV envelope proteins, and especially to the small S antigen. Relatively rare breakthrough events were mainly observed in Taiwan, Singapore, China, Italy, Japan, and Alaska, which have achieved remarkable control of HBV through massive immunization campaigns. Breakthrough infections were reported in immunized infants born to HBsAg-positive mothers receiving active or passive/active immunization as well as in liver transplant recipients treated with HBV polyclonal or monoclonal immune globulin.3, 4, 5, 6, 7 The main risk factors include wild-type intrauterine HBV transmission and high viral load in HBsAg+/hepatitis B e antigen-positive pregnant women who transmit HBV to their newborns despite passive/active or active immunization; immunologic selection pressures through the host or vaccine induced immune responses (including hepatitis B immune globulin); treatment with nucleoside analogs leading to mutations in the HBV polymerase gene, which overlaps with the envelope gene coding for the “a” determinant of HBsAg; immune suppression; nonresponse to vaccination as well as spontaneous emergence of mutations owing to the error prone function of the viral polymerase.4 Mutations at the B- and T-cell epitopes of the envelope genes lead to substitution of amino acids within the “a” determinant of HBsAg, which generate a conformational change affecting the binding of HBsAg to neutralizing antibodies. The most frequent substitution is the sG145R mutation in the S gene, defined by a glycine to arginine switch. Additional mutations, namely, at positions 124, 126, 137, 143, and 144, have been described.5 Although the sG145R mutant has been shown to be replication fit and infectious in chimpanzees even in the presence of protective anti-HBs antibodies, it readily reverted to the wild-type HBV sequence.8 In contrast, stable sG145R mutants and a high viral load were also shown to persist for years in patients with CHB. Yet, the clinical significance of such mutations and their practical public health implications has been questioned repeatedly over the years. In view of the outstanding success in the global control of HBV infection through universal vaccination of newborns, emergence of sporadic breakthrough infections after vaccination was often considered anecdotal and of limited public health implications. Taiwan was the first country to introduce mass and then universal vaccination in newborns.9, 10 Consequently, between 1984 and 1999, the HBsAg carrier rates in Taiwanese children dropped from 9.8% to ∼ 0.7%. During that period, prevalence of S mutants among HBsAg-positive children increased after the introduction of universal vaccination from 7.8% to 23%.11 However, in 2010 epidemiologic surveys failed to detect an increased rate of vaccine escape mutants (VEMs) in children and adolescents fully immunized over a period of 20 years.12 After its successful immunization effort, Taiwan became one of the gate keepers for surveillance of VEMs, including the confirmed VEM sG145R.

In the present issue of Gastroenterology, Lai et al13 report an increasing seroprevalence of anti-HBc and/or HBsAg as well as HBV-DNA in an older group of vaccinees in Taiwan, immunized 18–21 years ago. Individuals who were vaccinated during their childhood were grouped into 3 cohorts by age at 10–14, 14–18, and 18–21 years. Seroprevalence of HBV markers detected in 60 of 1214 individuals selected from various regions in Taiwan, was highest in the 18- to 21-year-old cohort (born after July 1986) among whom 2% were HBsAg+, 8.1% anti-HBc+, and 3% had detectable HBV-DNA. In comparison, prevalence of these markers was 0%, 0.4%, and 0%, respectively, in the 10- to 14-year-old cohort. Vaccination coverage was ∼76%, of whom ∼70% received ≥3 doses, rising to 92.7% in the youngest cohort. Interestingly, in the oldest cohort, prevalence of HBV-DNA was higher in individuals completely vaccinated compared with those who did not receive all recommended doses. Among the 60 of 1214 vaccinees with detectable anti-HBc or HBsAg, 10 were HBV-DNA positive and 5 of those 10 had substitutions in HBsAg. The overall prevalence of variants was 0.45% in subjects born after 1984, rising to 2.63% in those >18 years old, compared with 0.10% in those <18 years old. Univariate analysis identified rising age of vacinees as a predictive risk factor for the 3 seropositive markers. Multivariate logistic regression analysis suggests that anti-HBc positivity was independently associated with older age and incomplete vaccination. The investigators interpret their findings as an alarming increase in the rate of possible VEMs especially among 18- to 21-year-olds who received a full course of the vaccination. They conclude that the rising numbers of escape mutants in adolescents and young adults are a cause for concern, with potential implications for public health strategies for the prevention of HBV infection.

A rising prevalence of anti-HBc, HBsAg, and HBV-DNA in various combinations as well as the detection of possible VEMs in individuals vaccinated >18 years ago (Figure 1)13 may indeed be a cause for concern. Yet, currently available, low-cost, yeast-derived HBV vaccines have an outstanding record of safety and immunogenicity and remain the backbone for HBV prevention programs worldwide. However, these vaccines do not guarantee complete protection against confirmed VEMs. Although it is unlikely that this phenomenon will have an immediate impact on public health policies, a contingency plan should be prepared in case such a VEM outbreak becomes a reality. At present, it is not known whether third-generation, highly immunogenic PreS/S HBV vaccines14 will be protective against confirmed VEMs and this awaits clinical confirmation.

In addition to the analysis described, the report provides a wealth of information regarding the impact of universal vaccination on HBV seromarkers in a country that was previously endemic for HBV. One important observation is the falling levels of anti-HBs antibodies over time in the older cohorts of vaccinees. Many of these individuals are expected to generate an anamnestic response after challenge with wild-type HBV or a booster vaccine dose (which is at present not recommended in individuals who developed adequate seroprotection post immunization). However, loss of immune memory in vaccinees over time is by now recognized and must be closely followed, especially in defined risk groups such as health care workers involved in exposure prone procedures. This risk is reflected in the present study in which 5 of 10 HBV-DNA positive vaccinees developed wild-type HBV infection of who 3 received in the past 3–4 doses of an HBV vaccine.

The data reported by Lai et al are also open to different interpretations. Considering the magnitude of the vaccinated cohort in Taiwan, the detection of only 5 so-called VEMs does not necessarily predict a public health catastrophe. Furthermore, there is considerable doubt as to whether cases 1, 4, 5, and 8 (described in Table 3 of the paper) are indeed VEMs. To the best of our knowledge, none of the variants reported for cases 1, 5, and 8 have been proven without doubt to be VEMs. In this context, one should also distinguish the sG145A variant with glycine to alanine substitution found in case 8 (which would have minimal conformational impact on the “a” determinant and anti-HBs binding) from the confirmed VEM sG145R. As for the remaining cases (4 and 5), the small S variants sE164V and sL216 have so far not been clearly shown to be VEMs. Finally, the profile provided for case 2 may also be interpreted as transmitted lamivudine resistance or lamivudine resistance in a patient on active therapy. The investigators state that the 1214 serum samples tested were received from various districts in Taiwan participating in a concurrent epidemiologic study. However, no information is provided regarding the methodology of subject's selection. Furthermore, 3 of the 5 individuals with reported VEMs had a very low viral load detected only by ultrasensitive polymerase chain reaction. Although the investigators employed proofreading technology to prevent polymerase chain reaction–induced substitutions, the 100-microliter, low volume of serum used for analysis raises a concern regarding the possibility of false-positive results, especially in individuals with borderline HBsAg and undetectable anti-HBc. Finally, although 4 of the 5 possible VEM subjects had borderline elevation of aspartate aminotransferase or bilirubin, the clinical implications of the low-level VEM and the long-term impact on the natural history of CHB remain unknown at present and require longer follow-up.

In conclusion, the study results confirm the impressive reduction in prevalence of overt and occult HBV infection in Taiwan after the introduction of universal vaccination for newborns and catch-up immunization in older children after 1986. The emergence of confirmed VEMs such as sG145R does require increased surveillance, especially in highly populated countries, endemic for HBV, and involved in mass vaccination. However, in view of the doubts regarding the clinical significance of the observed mutants and their linkage to vaccination, agreement must be reached regarding an international consensus definition of VEMs. Information reported3, 4, 5, 15, 16, 17 will be a good starting point for such a dialogue. Achieving a consensus requires the development of validated assays for evaluating the infectivity and clinical significance of the individual variants claimed to be VEMs. Based on the evidence provided in the report of Lai et al, it is the opinion of these authors that it is premature to call the situation in Taiwan alarming. Meanwhile, efforts must continue to increase the vaccine coverage rates among newborns in endemic countries and enforcement of the 3-dose vaccine schedule.

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Key Concepts


•Current vaccination programs for hepatitis B remain highly effective in reducing the burden of HBV and ongoing transmission in high prevalence areas such as the Far East, especially in the first 2 decades of life.

•As more and more countries engage in evaluation of their HBV universal vaccination programs, it will be essential to map the molecular epidemiology of wild-type as well as variant HBV strains.

•In this context, there is a need to establish an international consensus regarding the definition and evaluation of claimed, as well as confirmed vaccine escape variants and their public health implications.

•Recent evidence suggests that HBV vaccine-induced anti-HBs levels are waning in immunized persons entering into young adulthood who participated in universal immunization programs.

•Linked to these waning anti-HBs levels may be the risk of transmission of HBV in situations associated with high-risk exposure to HBV, that is, via young adult behavior such as unprotected sexual intercourse and/or injecting drug use or in healthcare workers engaged in exposure prone procedures. Consequently, guidelines regarding administration of booster vaccination may have to be reevaluated.