Review of long term immunogenicity and tolerability of live hepatitis A vaccine

ABSTRACT

Hepatitis A represents one of the major public health problems worldwide including India. Vaccination is the most effective way to prevent hepatitis A infection. Two types of hepatitis A vaccines-live attenuated (H2 strain) and inactivated (killed) are available for use in clinical practice in India with former having advantage of a single-dose compared to two-dose killed vaccine. One of the important characteristic of an ideal vaccine includes its ability to provide life-long protection. In this article we reviewed the available long-term (≥10 years follow-up) published data on live attenuated hepatitis A (H2 strain) vaccine. The data from country of origin of the vaccine (China) and India establish the long-term immunogenicity, protection, and tolerability. Based on the results of several clinical trials showing long-term protection, single dose of live attenuated hepatitis vaccine can be widely used to protect high-risk population against hepatitis A virus infection and related complications.

KEYWORDS:

• hepatitis A vaccine
• hepatitis A vaccine
• immunogenicity
• live attenuated
• tolerability

Introduction

Hepatitis A represents the most common form of acute viral hepatitis.¹ It is transmitted through feco-oral route. Feces of infected people act as the major source of infection. Causes of transmission include contaminated water and/or food and poor sanitation and hygiene. Close contact with an infected person increases the risk of hepatitis A virus (HAV) transmission. The HAV can tolerate extreme temperature and survive outside the body for months. High temperatures, such as boiling or cooking food or liquids for at least 1 minute at 185°F (85°C), are required to kill the virus. Good hand hygiene practices like washing hands after using the washroom, changing diapers, and before preparing or eating food are vital in preventing the spread of hepatitis A. However, the most effective way to prevent hepatitis A is by vaccination with the hepatitis A vaccine.²

The risk of hepatitis A infection is associated with socio-economic conditions, personal hygiene, and access to safe potable water.³ India is epidemiologically transiting from high to intermediate endemicity with coexisting pockets of high- and low-incidence regions. Till the time we are able to further reduce this endemicity by improving hygiene and socio-economic conditions, vaccination can be used as an important tool in changing the epidemiology of HAV infections in India. An epidemiological study by Arankalle et al.⁴ has also shown the trend of increasing seroprevalence of HAV infection in older-age children and the need for vaccination in this heterogeneous Indian scenario.

Currently, two types of hepatitis A vaccines are available in India; formaldehyde inactivated and live attenuated vaccines.⁵ The inactivated (killed) vaccine requires two doses at least 6 months apart. The live attenuated vaccine, developed in China, has shown long-term immunity after single dose.⁶ Here we discuss further the long-term immunogenicity and tolerability of live attenuated hepatitis A vaccine (H2 strain).

We searched the published literature from “PubMed” to screen the studies, which are suitable for inclusion in this review. Studies with follow up of 10 years or more are included in the review.

Long-term immunogenicity and tolerability of live attenuated hepatitis A vaccine (H2 strain)

Some of the characteristics of an ideal vaccine include its ability to provide lifelong immunity, protection against all variants of the pathogen, rapid and effective responsiveness, immunogenicity in all age groups, protection with even a single-dose administration, minimal adverse reaction potential, good tolerability, and cost-effectiveness.⁷ One of the important parameters for performance of a vaccine is the duration of immunity provided. Ideally, single dose of vaccine should provide lifelong or at least few decades of immunity against the disease. Live attenuated hepatitis A vaccine is one of those few vaccines providing long-term protection with a single dose.⁵ It is commonly used in China⁸ and India. Seroconversion occurs in about 3 weeks after vaccine administration. No significant reduction in the titer of HAV antibody was seen after one year of administration.⁹

Different serological assays [e.g. quantitative Axsym HAVB 2.0 enzyme-linked immunosorbent assay (ELISA), AxSYM HAVB 2.0 microparticle enzyme immunoassay, electrochemiluminescence immunoassay (ECLIA), etc.] are used for the assessment of immunogenicity.¹⁰ Antibody levels are expressed in mIU/mL unit. ECLIA has shown higher sensitivity than ELISA-based immunoassays.¹¹ Hence, assessment of antibody titers may vary based on the serological assay used. Readers should consider this point while interpreting the results. Seroprotection rate is defined as total anti-HAV antibody level of 20 mIU/mL or more.^6,10,12

Many studies reported long-term effects (follow up of 10 years of more) after administration of live attenuated hepatitis A vaccine. Table 1 summarizes these long-term studies on live hepatitis A vaccine.

Table 1. Seroprotection rate reported in long-term studies with live attenuated hepatitis A vaccine

Author	Age and number of participants	Duration of follow up	Dose	Serological assay(s) used	Seroprotection rate at the end of study
Zhuang et al.^Citation13	1–15 years (n = 30908)	10 years	Single-dose injection	ELISA Some serum was assessed by Abbott EIA and IMX automatic detector	80.2%
Zhuang et al.^Citation14	1–3 years (n = 220)	10 years	Single-dose injection	ELISA Some samples were randomly assessed by ABBOTT Axsym HAVAB 2.0 microparticle enzyme immunoassay (mEIA)	80.2%
Zhuang et al.^Citation12	1–3 years (n = 220)	15 years	Single-dose injection	ELISA and ABBOTT Axsym HAVAB mEIA	81.3%
Bhave^Citation10	3.5–15 years [n = 143 (121 evaluated of which13 were failures)]	10 years	Single-dose injection	ECLIA	98.1%
Chen Y et al.^Citation6	1–12 years (n = 3515)	17 years	Single-dose injection	Serum anti-HAV antibody measurement (ELISA). Memory B Cells (ELISPOT) and T cells (Flow Cytometry assay)	62% After booster challenge dose, approximately 28 fold and 83 fold increase in antibody titers in anti HAV positive and anti HAV negative groups, respectively

Chinese study (10-year follow-up) – Zhuang et al¹³

A mass vaccination program with live attenuated (H2 strain) hepatitis A vaccine was initiated on 1–15-year-old children in different provinces of China in 1990–1991. A pilot project was conducted in a province with high incidence of hepatitis A. The project aimed at assessing the long-term (10 years) epidemiological effects of using live attenuated hepatitis A vaccine especially in high incidence areas. The cohorts were selected from Shengsi County (n = 17,285) and Jiaojiang district of Taizhou city (n = 13,623), both in Zhejiang province. Immune recall reaction and neutralization tests were also carried out 6 years after initial vaccination apart from studying other epidemiological parameters.

The immune effect of the vaccine in terms of incidence rate of hepatitis A in Shengsi county in 4 years (1990–1993) was found to be significantly different (p < .05) in vaccinated group compared to unvaccinated group (0% vs. 2.32%). The vaccine protection rate was found to be 100% during the complete 10-year follow-up. The incidence rate showed a reciprocal relationship with vaccination rate among children 1–15 years old. The average incidence in this population decreased from 1171.8 cases and 32 cases per million in 1983–1989 in Shengsi and Jiaojiang respectively to 0 cases per million in 1999, in each of these places. Rise in vaccination coverage from 57% to 74% showed a complete control on hepatitis A epidemic, while at vaccination coverage of 85%, there was absence of any case of hepatitis A.¹⁴ The vaccination rate reached almost 90% in these two regions during this 10 year observational period. With such a high vaccination rate, hepatitis A incidence rate decreased by more than 90% in whole population in both regions, including unvaccinated adult population, suggesting development of herd immunity.

Recall response was studied by reinoculation with vaccine 6 years after initial vaccination in seronegative candidates (n = 61). There was a significant elevation (p < .01) in HAV–IgG levels seen with reinoculation. This showed the obvious recall reaction of live attenuated hepatitis A vaccine.

The serological tests were done by using ELISA. Part of the sera was tested with Abbott EIA and IMX automatic detector. Neutralizing antibody test was carried out by diluting HAV to 31.6 TCID50/mL with ELISA negative postvaccination serum. 63.80% of the ELISA negative (for HAV–IgG) sera were still able to neutralize HAV. ELISA kit has shown to have sensitivity of 50–80 mIU/mL and specificity of 97–100%, while Abbott EIA had sensitivity of 20 mIU/mL and specificity of 100%.¹⁴ The study highlights the differing sensitivity of detection methods used for labeling a serum positive for protective antibodies.

The seropositivity 10 years after vaccination was found to be 80.2% with no hepatitis A occurring in any of the vaccinated person during this observational period. This seems to be the first study to show that live attenuated hepatitis A vaccine affects positively the long-term epidemiological parameters.

Chinese study (10-year follow-up) – Zhuang et al¹⁴

A total of 220 children aged 1–3 years were selected for serological assay from 2 care centers which were part of mass vaccination program from Jiaojiang city in Zhejiang province as detailed in the above mentioned study.^9,13 This study aimed at evaluating immune response in terms of seroconversion, population immunity over time, and persistent immunity of live hepatitis A vaccine (H2 strain) after its inclusion in mass vaccination program. The eligibility criteria included subjects with negative serum anti-HAV (IgG) and normal alanine aminotransferase levels. Follow-up with serum sample collection was done at 0 month, 2 months, 12 months, 6 years, and 10 years after administration of single dose of live attenuated hepatitis A vaccine (H2 strain).

Simultaneously, a serological survey was conducted to assess the correlation between live hepatitis A vaccine coverage and hepatitis A morbidity. Subjects for this survey were randomized and sampled based on ‘7 digital principle’ as recommended by WHO. Subjects were randomized in seven different age groups of 3, 6, 9, 15, 18, 25, and 35 years. A total of 100 subjects were chosen from each group. Initial sampling was done before the start of vaccine program in the community (March/April 1991) and another sampling 10 years later (May/June 2001).

Serological follow-up for seroconversion rate at 2 months, 12 months, 6 years, and 10 years in vaccine recipients was 98.6% (GMT 287 mIU/mL), 93.6% (GMT 226 mIU/mL), 83.3% (GMT 173 mIU/mL), and 80.2% (GMT 145 mIU/mL), respectively. Differences in accuracy of the serological testing methods need to be considered while reviewing data as the results of this study are also based on ELISA technique. Also the samples showing anti-HAV antibody titer >1:100 by ELISA were considered unlikely because of vaccine effect and thus excluded from analysis. However, there was subtle decrease in seropositivity and GMT in vaccine recipients and the level remained much higher than the minimum required protective levels during these 10 years of observation.

Serological survey before the vaccination program in 1991 showed a gradual increase in seropositivity rate in the seven surveyed age groups. The seropositivity was low at 7.69% in children aged 3 years, increasing up to 89.42% in those aged 35 years (6 years – 19.19%, 9 years – 21.82%, 15 years – 49.06%, 18 years – 52.58%, and 25 years – 68%). However, vaccination in the community led to a high seropositivity rate in early age group and same was found to be maintained over all age groups. The seropositivity rate was 70.54%, 61.82%, 63.87%, 65.83%, 71.78%, 77.14%, and 92.08% in age groups 3, 6, 9, 15, 18 25, and 35 years, respectively.

Chinese study (15-year follow-up) – Zhuang et al¹²

Zhuang and colleagues¹² continued the follow-up of 220 children aged 1–3 years for another 5 years, after the 10 years analysis, thus accumulating the total data of 15 years. The seroconversion rate and postvaccination GMT was found to be 81.3% and 128 mIU/mL respectively in this 15 years follow-up study. The results further extend the overall protection rate of 100% for 15 years. Thus, single dose of live attenuated hepatitis A vaccine confers protection against hepatitis A infection even after 15 years of vaccination in children.

Epidemiological observations were also continued to be noted during these 15 years. There was absence of occurrence of any case of hepatitis A in an observational group of 20661 vaccinated people aged 1–29 years (1–15 years age group getting vaccinated in last 15 years), corresponding to 236,413 man years. As against this, another group of 2745 nonvaccinated people in similar age group, corresponding to 27,206 man years, showed occurrence of four cases of hepatitis A (2 cases each in 2002 and 2007). The data further affirms 100% vaccine protection rate as seen in previous follow-up results.

Chinese study (17-year follow-up) – Chen et al⁶

A randomized trial⁶ aimed at assessing the efficacy of live attenuated hepatitis A (H2 strain) vaccine was conducted between 1996 and 1999. 3515 HAV susceptible children aged 1–12 years from 24 villages received a single dose of live hepatitis A vaccine or were assigned as controls in this trial. A longitudinal cohort study followed-up these recipients of vaccine for 17 years after initial administration. We were unable to find the details of this cohort study in published literature, while the results of the randomized control trial are beyond the scope of this review.

However, a serological follow-up and immune memory of the single dose of live attenuated hepatitis A (H2 strain) vaccine in a subset of the cohort population was studied separately. Recipients of the vaccine 17-years ago from two villages (n = 51) were identified for this study. Vaccine recipients with either natural infection, reflected by anti-HAV antibody titer >10,000 mIU/mL, or vaccine booster (defined as either receiving traceable additional dose of HAV vaccine, or twofold increase of anti-HAV titers compared to that from last measurement during 17-year serological follow-up) were excluded from the study. As such, the study was conducted on 47 participants. A booster challenge dose of live attenuated hepatitis A (H2 strain) vaccine was administered to simulate the natural virus exposure and the immune cell recall response was assessed. A total of 31 among the 47 participants agreed to receive this booster dose.

Anti-HAV antibody level

About 29 (62%) out of 47 participants were found to be seropositive even after 17-years of initial vaccination. 20 mIU/mL is considered as seroprotective level for hepatitis A disease. Participants with titers above 20 mIU/mL were considered in anti-HAV positive group and below 20 mIU/mL were considered in anti-HAV negative group. The GMTs of anti-HAV serum IgG were 64.8 and 7.6 mIU/mL in seropositive and seronegative groups, respectively.

Of the 31 participants who agreed to receive booster, 13 were anti-HAV negative. The GMT in these participants increased from 8 to 663 mIU/mL. The GMT post-booster in anti-HAV negative participants was close to 10 times higher than GMT of anti-HAV participants before booster (664 mIU/mL vs. 67.6 mIU/mL). The GMT in anti-HAV positive participants receiving the booster dose increased from 67.6 to 1832.1 mIU/mL. Anti-HAV antibody was thus significantly elevated in both seropositive and seronegative participants (28 times and 83 times, respectively) just 2 weeks after receiving the booster dose given to simulate natural virus exposure.

Longevity of memory B cells

There was a significant increase in IgG-secreting cells due to the impetus provided by HAV Ag (booster dose) compared to controls not receiving the exposure in the form of booster dose. Additionally, there was no difference in the frequency of HAV-specific memory cells in seronegative and seropositive participants receiving the HAV Ag exposure.

Recall responses of memory B cell after Ag exposure

A typical anamnestic response was seen in the form of rapid increase of anti-HAV serum IgG GMTs from 67.6 to 1832.1 mIU/mL (approximately 28 fold) and 8 to 661.3 mIU/mL (approximately 83-fold) in seropositive and seronegative groups respectively after the exposure to HAV Ag from booster challenge dose. In vivo proliferation of anti-HAV IgG-secreting B cells was measured and found to be significantly greater post-HAV Ag exposure compared to prechallenge with booster dose (p < .0001). This rise in proliferation was similar in seropositive and seronegative participants.

Persistence of memory T cells

Cell-mediated immunity in the form of cytokine-secreting T cells from the peripheral blood mononuclear cells (PBMCs) was measured. The frequency of CD4+/CD8+ memory T cells secreting the cytokines IFN-γ/IL-2 was assessed. The frequency of T cells secreting cytokines was found to be significantly higher in Ag stimulated cells postchallenge compared to unstimulated cells. No difference in the magnitude of cytokine secretion was seen in seropositive and seronegative participants.

Recall responses of memory T cells after the Ag exposure

HAV-specific T cell responses in the PBMCs with in vitro testing were found to be significantly higher in the Ag exposed and stimulated group compared to unstimulated group. T cell subsets increased drastically postchallenge compared to prechallenge.

Overall, this 17-year follow-up study clearly suggests the role of memory cell functions and anamnestic response in providing long-term protection to the recipients of live attenuated hepatitis A vaccine. Responses from memory B and T cells suggest that protection after vaccine administration may be present despite the undetectable level of anti-HAV antibodies.

Indian study (10-year follow-up) – Bhave et al¹⁰

Several studies^10,15,16 among Indian subjects have demonstrated immunogenicity and safety of live attenuated hepatitis A vaccine. One such study was conducted in 2004 at KEM hospital and research center in Pune. The subjects in this study are being assessed annually and have recently concluded the 15^th year of follow-up. The 10-year data of this on-going study was analyzed and published in 2015.¹⁰ Initially live attenuated hepatitis A (H2 strain) vaccine was administered as a single dose in 143 children in 2004. Among these, 121 children reported for the ten year follow up in 2014; 13 children were not included in the analysis because they were either vaccine failures and received two doses of inactivated (killed) HAV vaccine or had received a second dose of live vaccine due to attainment of titer lower than the WHO recommendation (<20 mIU/mL).

None of the 108 analyzed children developed hepatitis-like illness in the last 10 years. The seroprotective titer of IgG antibodies (>20mIU/mL) was seen in 98.1% participants. None of these children showed presence of anti-HAV IgM antibody. Even after 10 years, the GMT of anti-HAV antibodies among seroprotected children was 100.5 mIU/mL, 5 times more than the minimum seroprotective level of 20 mIU/mL. The efficacy of vaccine considering all 121 subjects under analysis was found to be 87.6%. This study shows that single dose of live attenuated vaccine can provide long term immunity in Indian population.¹⁰ The study did not report any significant safety or tolerability concern over a period of 10 years. These results confirm the immunogenicity, efficacy and safety of live Hepatitis A vaccine in Indian children, even after 10 years postadministration of single dose.

Results from ongoing follow-up of these participants will further establish the long-term immunogenicity and safety of live attenuated hepatitis A vaccine in Indian population.

Safety and tolerability of live hepatitis A vaccine

The results of studies^6,10,12–14 reported in this review do not show any significant clinical safety concern. Moreover, although authors did not categorically mention it, but post booster safety of live vaccine seems to be similar to that of primary vaccination.⁶ WHO position paper (2012) in its review of studies on live hepatitis A vaccine (H2 strain) also stated that there are no considerable safety concerns seen with the vaccine.⁵ Evaluation of immediate or short term local and systemic adverse effects are beyond the scope of this review as published studies of 10 year and more follow-up duration are solely considered.

Duration of protection

Data from Chinese studies show continued protective levels of anti-HAV antibody titers after 17 years of initial administration of live hepatitis A vaccine in 62% recipients. The Indian study on single dose of live hepatitis A vaccine has completed 15 years of follow-up. Results from this follow-up will be quite useful in establishing the duration of protection in Indian population. Data from 10-year follow-up in Chinese as well as Indian studies has shown >80% protection with this vaccine.^10,13,14 There is a role of anamnestic response despite low titers. The 17-year follow-up study⁶ establishes the effect of this anamnestic response in the form of substantial increase in anti-HAV antibody titers after exposure to HAV (booster dose of vaccine) in seropositive as well as seronegative subjects.

Killed/inactivated hepatitis A vaccines have reported a protection of >97% at 20 years of follow-up. The mathematical models have predicted ≥90% subjects will remain seropositive at year 40.¹⁷ However, there is no published data on inactivated hepatitis A vaccines in Indian population to compare it with data of live hepatitis A vaccine from Indian origin.

Moreover, WHO position paper (2012) concluded that both, live and inactivated, hepatitis A vaccines provide long-lasting or possibly lifelong protection against hepatitis A disease in children and adults.⁵

Status of live attenuated hepatitis A (H2 strain) vaccine in vaccination programs and recommendations

In China live attenuated hepatitis A vaccine is in use since 1992 and is a part of Expanded Program of Immunization since 2008.¹⁸ Apart from China and India, the vaccine is also available in Thailand, Philippines, Guatemala, Nepal, Uzbekistan, Chile, and Bangladesh.^19,20

The long term studies demonstrate the durable immunogenicity, efficacy, and safety of single-dose live attenuated vaccine. Based on scientific evidence, several countries have included single-dose live attenuated hepatitis A vaccine in the national immunization program for children with age of one year or more.⁵ WHO position paper on hepatitis A vaccines⁵ also acknowledges the comparability of single dose of live hepatitis A vaccine with two doses of killed hepatitis A vaccine in providing long-term protection against HAV infection.

Indian Academy of Pediatrics (IAP) also recommends single-dose administration of live attenuated H2 strain hepatitis A vaccine starting at 12 months and through 23 months of age. Catch up vaccination in children beyond 2 years of age also requires single dose of live hepatitis A vaccine. However, prevaccination screening for hepatitis A antibody is recommended in children more than 10 years of age.²¹

Conclusion

Live attenuated hepatitis A vaccine has long history of development and research since more than three decades. It has been extensively studied for its immunogenicity, tolerability and long term protective effects. Promising immunogenicity, protection, and safety has been reported with single-dose injection of live attenuated hepatitis A vaccine in all the long-term follow-up studies. Recent study has reported pivotal information on development of immunological memory with single dose vaccination of live attenuated vaccine in the form of persistence of HAV-specific memory B- and T-cells. The long-term data on Indian population further establish the immunogenicity and tolerability of the vaccine and exhibit comparability to the data from country of origin of the vaccine. Based on the results of several clinical trials showing long-term protection, single dose of live attenuated hepatitis vaccine can be widely used to protect high risk population against HAV infection and related complications.

Disclosure of potential conflicts of interest

Archana Karadkhele, Gaurav Puppalwar and Rishi Jain are salaried employees of Wockhardt Ltd. Wockhardt Ltd has license to market live hepatitis A vaccine in India.