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Year : 2001  |  Volume : 19  |  Issue : 4  |  Page : 172--183

Immunoprophylaxis of hepatitis B virus infection

N Joshi, A Kumar 
 Department of Gastroenterology, Nizam's Institute of Medical Sciences, Punjagutta, Hyderabad - 500 082, India

Correspondence Address:
N Joshi
Department of Gastroenterology, Nizam«SQ»s Institute of Medical Sciences, Punjagutta, Hyderabad - 500 082
India

Abstract

Hepatitis-B infection is a global health problem. The spectrum of the disease is highly variable ranging from mild disease to chronic liver diseases including hepatocellular carcinoma. There are approximately 350 million chronic Hepatitis-B surface antigen (HBsAg) carriers in the world. Till date there is no effective therapy against this disease. Hence, prevention of the disease through vaccination is the only means to control the disease. Passive immunization is recommended for certain accidental exposures. Hepatitis-B immunoglobulin (HBIG) contains high titers of anti-HBs prepared from pooled plasma. HBIG has been shown to be highly effective in preventing post exposure transmission. HBIG induces immunity for a short period only hence, it is recommended to have a course of active immunization following passive immunization. Active immunization is achieved using vaccination. Two generations of vaccines, 1st generation plasma derived and 2nd generation recombinant DNA vaccines are available. Both these vaccines have been used extensively in all age groups all over the world. The studies have shown that HB vaccines are clinically well tolerated, safe and highly immunogenic. Normally 3 doses of HB vaccines are recommended in 0, 1, 2 and 12 or 0, 1, 6 months schedule. The dosages and schedules may vary in certain special groups, such as infants and neonates, chronic renal failure patients on hemodialysis. Advisory committee on immunization practices (ACIP) has given several guidelines regarding HB vaccination. Universal immunization of all infants and integration of HB Vaccine in the expanded program of immunization has been recommended by World Health Organization. Universal infant immunization is cost effective. Universal immunization of infants is the only strategy that will lead to the control and eradication of HBV infection in all regions of the world. Several countries have adopted this policy. But in India we have several problems in implementation of this policy. The high cost of the presently available vaccine is one of the major factors. The future consideration for hepatitis vaccines are focussed on multivalent combination vaccines with other childhood vaccines, and use of immunomodulators in conjunction with vaccine to increase the efficacy of vaccines in immunocompromised hosts.

How to cite this article:
Joshi N, Kumar A. Immunoprophylaxis of hepatitis B virus infection.Indian J Med Microbiol 2001;19:172-183

How to cite this URL:
Joshi N, Kumar A. Immunoprophylaxis of hepatitis B virus infection. Indian J Med Microbiol [serial online] 2001 [cited 2020 Jul 14 ];19:172-183
Available from: http://www.ijmm.org/text.asp?2001/19/4/172/8186

Full Text

Hepatitis- B infection is the major health problem all over the world. The disease is highly variable. The acute Hepatitis-B virus (HBV) infection may be mild, self limiting or it can cause fatal fulminant or sub fulminant hepatic failure in a small percentage of infected persons. The more serious consequences of chronic carrier state are progression to chronic liver disease namely chronic hepatitis, cirrhosis of liver and hepatocellular carcinoma. There are presently about 350 million chronic carriers of HBV in the world population.[1] It is one of the major killer diseases of mankind through the chronic sequelae it produces. It is estimated that 75-100 million of them will die of liver cirrhosis and/or hepatocellular carcinoma. The probability of developing the carrier state following HBV infection is greatest in early life and decreases with increasing age. Upto 90% of babies born to carrier mothers may become carriers and they are at a very high risk of developing chronic liver disease at a younger age.[2]

Control of HBV infection has therefore been recognized as a priority all over the world. There is no specific and effective therapy against this dreadful disease, hence prevention of the disease through vaccination remains the only method of choice for its control and eradication.[2],[3]

 Prevention of HBV Infection:



Passive Immunization against Hepatitis-B:

Wherever possible, immunization before exposure to HBV is preferred. However, there are a number of settings in which post exposure prophylaxis in the form of passive immunization alone or in conjunction with hepatitis B vaccine is either necessary or desirable. Postexposure prophylaxis is indicated in the following settings.

1) Sexual exposure with Hepatitis B surface antigen (HBsAg) positive person.

2) Accidental percutaneous or permucosal exposure to HBsAg positive blood.

3) Perinatal exposure from HBsAg positive mother.

Before the advent of vaccine, passive immunization with anti-HBs was the sole option. Hepatitis B immunoglobulin (HBIG) is prepared from pooled plasma with high titers of Hepatitis-B surface antibody (anti-HBs). The HBIG has an anti-HBs titer of > 100,000 by radioimmunoassay. The human plasma from which HBIG is prepared is screened for antibodies to Human Immuno Deficiency Virus (HIV), in addition the process used to prepare HBIG inactivates and eliminates HIV from the final product. HBIG confers temporary passive immunity in certain pre and post exposure situations.

A single or two injections of HBIG (0.06 mL/kg I.M) given one month apart have shown protection against both clinical disease and subclinical infection both, for sexual contact and accidental needle prick exposure.[3]

The most important indication for the use of HBIG is for infants born to HBsAg positive mothers. In various studies it is shown that passive active immunization in the form of a single dose of 0.5 mL HBIG at birth or within 12-24 hrs of exposure in conjunction with three doses of 10 µgms of hepatitis B vaccine at 1, 2 and 6 months resulted in better and long term protection than passive immunization alone.[4]

Recent studies with currently available yeast recombinant hepatitis B vaccine also suggest that similar levels of protective efficacy may be obtained using regimens of vaccine only without concomitant HBIG.[5] This is particularly useful in high endemic areas where it is not feasible to screen women for HBsAg or to employ combination of active and passive immunization. The cost of the HBIG is also very high for mass use.

 Active Immunization:



Active immunization of HBV is achieved by hepatitis B vaccine. Currently there are two types of vaccines available. The first generation Plasma derived vaccine and the second generation Recombinant vaccines.[6],[7] The first HBV vaccine for routine use became available in 1981 in the United States. The production process for this vaccine was state of the art and unique to any vaccine at that time, given that the biological material was highly infectious HBsAg positive plasma from chronic carriers.

The basis for the development of inactivated hepatitis B vaccine stemmed from i) The discovery of the Australia antigen. ii) It's subsequent identification as HBsAg and iii) the demonstration that heat-inactivated serum containing HBV and HBsAg was not infectious but was immunogenic and partially protective against subsequent exposure to HBV. Vaccine development was accelerated in 1973 when hepatitis-B infection was successfully transmitted to chimpanzees.[8],[9]

Plasma derived vaccines:

Plasma from carriers of HBV contains varying amounts of 22nm HBsAg particles (spheres or tubules), Dane particles (Virus) and normal human plasma components. The HBsAg in plasma derived vaccines is purified by physicochemical methods (eg. Chemical precipitation, ultra-centrifugation, column chromatography etc.) and is treated with chemicals (eg. pepsin, urea, formalin heat etc.) to inactivate potential contaminating infectious agents including HIV. The purified surface antigen consists of a homogenous suspension of 22nm particles consisting of 'S' antigen alone and free of pre-s component that may be present in small and variable amounts in native surface antigen. Testing of these vaccines in chimpanzees and humans demonstrated that they were safe, immunogenic and highly effective in preventing HBV infection.9-11 These studies led to the licensure and use of the first plasma derived vaccine in United Sates in 1982. Despite its excellent efficacy and safety profile in millions of individuals worldwide, acceptance of the vaccine was slow. The process used was cumbersome and time consuming with the manufacturing cycle as long as one year. Furthermore availability of HBsAg positive donor's plasma has limited the production of vaccine for use worldwide. Therefore a need was recognized for a second generation Hepatitis-B vaccine.

Recombinant vaccine:

The recombinant vaccines are produced by transfer of the portion of HBV genome, the 'S' gene coding for HBsAg to an appropriate plasmid that is then inserted into the desired expression vector. The development of recombinant vaccine goes through a number of steps [Figure:1]

The master seed thus produced is amplified in fermentation culture. The fermentation may last from several days to 1-2 weeks, depending on the scale and cell inoculum size to achieve maximal HBsAg productivity. Since HBsAg accumulates within recombinant yeast cells, the final production fermentor is harvested and the cells are separated from the medium by filtration, centrifugation and are washed. Cells may then be stored or frozen or immediately lysed for HBsAg purification. HBsAg is purified by chromatography and ultracentrifugation. The recombinant vaccine consists of the purified antigen adsorbed onto an adjuvant, usually aluminum hydroxide. Purity is judged on the basis of HPLC analysis as well as SDS- PAGE with coomassie staining and is reported to be >98%. Identity and potency are confirmed by animal models as well as by Enzyme Immuno Assay (EIA) or Radio Immuno Assay (RIA) tests. Concentrations for dosage determination is measured by Lowry protein with potency confirmed by EIA and mouse potency tests. The released bulks are filled aseptically and released in final dosages form, after appropriate testing for pH, sterility, safety, potency etc.[7],[12]

The characteristics of both generations of vaccines are shown in [Table:1]. There are subtle differences between the plasma-derived and recombinant vaccines. Vaccines that used yeast as the expression vector were licensed in United States in 1986 & 1989 and gradually replaced plasma derived vaccines. A vaccine based on HBsAg expressed in mammalian cells Chinese Hamster Ovary (CHO) is in use in Peoples Republic of China. The advantage of the recombinant vaccine is the unlimited supply, as no blood products are involved and a shorter production cycle.[12],[13]

The first genetically engineered vaccine produced in India (Shanvac-B manufactured by Shantha Biotechnics Pvt. Ltd., AP) was made available in 1998. The safety and immunogenicity of the vaccine was assessed in various clinical trials and the vaccine was also compared to the commercially available vaccine. The results have shown that the indigenously developed HBV vaccine is highly immunogenic, clinically safe, and well tolerated. Now there are few more vaccines, which are developed and available in India.[14],[15],[16],[17]

Safety of Recombinant Hepatitis-B Vaccine:

Recombinant yeast hepatitis-B vaccines have undergone extensive evaluation by clinical trials. The results indicate that this vaccine is safe, antigenic and free from side effects (apart from minor local reaction in a small proportion of recipients). The immunogenicity and reactogenicity is similar to that of the plasma derived vaccine.[13] Recombinant hepatitis-B vaccines are now being used in many countries and have largely replaced plasma derived vaccine.

Recombinant vaccines have been given to thousands of individuals in clinical trials conducted in the United States and to millions of people throughout the world. The most frequent side effects observed are injection site soreness and mild fever. The other adverse reactions included mild fever, local tenderness or systemic reactions such as headache, dizziness, gastrointestinal symptoms, malaise, fatigue etc. These and other reactions generally resolve within 24-48 hrs of vaccine administration and are less frequent with subsequent doses. Reactions are less frequently seen in infants and children than in adults. Severe adverse events have been reported infrequently in adults and rarely in infants and children. A possible association was found between reported cases of Guillain-Barre syndrome and recipients of plasma derived vaccine in adults.[18] This association has not been found with recombinant vaccine (center for Disease control and prevention unpublished data) and has not been observed among children given hepatitis-B vaccine.[19] The various clinical trials on indigenous vaccine have also shown high safety profile and clinical acceptability.[14],[15],[21]

Immunogenicity:

The immunogenicity of Hepatitis-B vaccine is generally evaluated according to three parameters 1) Seroconversion rates 2) Seroprotection rates and 3) Geometric mean anti HBs antibody titres (GMT). The seroconversion is defined as an increase in antibody titer (anti-HBs) from an undetectable amount to a level above the assay cut off which is usually 1 mIU/mL. Seroprotection is considered to have occurred if concentration of the anti-HBs antibodies reaches or exceeds 10 mIU/mL. The GMT is calculated using the log transformation of titers > 1mLU/ml for anti-HBs and taking the anti log of the mean of these transformed values.

A number of dose response studies have been done using either a 0-1 and 6 months or a 0,1,2 and 12 months schedule using both plasma derived and recombinant vaccines. Typically more than 90-96% of healthy adult vaccinees developed detectable anti-HBs titers irrespective of the schedule used. Although anti-HBs titers are consistently higher when third dose is given at month 6 rather than month 2. Seroconversion rates for anti-HBs and maximal titers of antibody achieved are proved to be age dependent. Children and young adults being more responsive to a given dose of vaccine than middle aged and older adults. The antibody response in neonates and children receiving 10 µg doses of vaccine were slightly higher than the response in adults receiving 20 µg doses. Infants, children and adolescents responded very well to lower dosages ranging from one quarter to one half of the adult dosage. 95-100% of children attain seroprotective levels of anti-HBs after 3 doses given at birth and at 1 and 6 months of age. Concurrent administration of HBIG at birth does not interfere with the infant's immune response to hepatitis B vaccine.[22]The seroconversion rates and GMT in adults (20µgms vaccine), neonates and children (10µgms vaccine) using 0,1 and 2 and 0,1 and 6 schedules are shown in [Figure:2a]and [Figure:2b].[13] Data comparing immune response according to age and sex showed that in general, adults over 40 years of age respond less than young adults and the GMT in females are higher than those in males although seroconversion rates are similar.[13],[23],[24] In one group of studies the protective seroconversion was 98% in young adults (20-29 years) compared to 89% in older adults (>40 years). Responders in younger age group had a GMT of nearly 1500 mIU/mL compared to approximately 500 mIU/mL in the older age group[20] Experience with different recombinant vaccines in children has revealed that GMT ranging from 294-314 m IU/mL was achieved at 4th month. The GMT increased to more than 20 fold with a booster at 12 months. Studies in infants and children (3 months to 10 years) demonstrated that the recombinant vaccines are highly immunogenic in this age group.[13],[20],[24] Seroprotection rate of nearly 100% and GMT greater than 1000 mIU/mL were attained in all groups after third dose. Our experience with 5µgms of Shanvac-B in infants and neonates revealed 100% seroprotection after the 3rd dose. The GMT attained was 1742mIU/mL.[25] Virtually all infants and children respond well to the recombinant vaccine irrespective of the schedule but when immunization is begun after 1 month of age higher anti HBs levels are attained.

Simultaneous Immunization with other vaccines:

Seven studies with more than 1000 children have assessed the comparative immunogenicity of hepatitis-B vaccine given with or without other childhood vaccines. Both the plasma derived and recombinant vaccines were used. The other vaccines given concurrently but at separate injection sites included diphtheria-tetanus pertusis (DTP), DTP inactivated polio, oral polio vaccine, measles, mumps, rubella (MMR) and yellow fever. In every study the investigators demonstrated that concurrently administered vaccines did not interfere with hepatitis B vaccine and vice versa.[26]

As per current recommendations HBV vaccine can be simultaneously given with DPT, Haemophilus influenzae type B, Polio and/or MMR vaccines but HBV vaccine should not be mixed in same syringe. Combined products are under evaluation. A combined tetravalent diphtheria, whole cell pertusis, tetanus and HBV vaccine are found to be safe and immunogenic with regard to each component.[27]

Efficacy:

The protective efficacy of Hepatitis-B vaccine can be assessed with respect to a variety of outcomes such as incidence of clinical disease, antigenemia (HBsAg positive with or without clinical manifestations of infection) or any HBV infection (including benign asymptomatic infection detected only through seroconversion to anti-HBs).

Vaccine efficacy is (E) is calculated as follows.

E (%) = 1 - (Incidence in vaccinees) x 100

(Incidence in controls)

Randomized double blind trials of the efficacy of plasma derived vaccines versus alum placebo enrolled homosexual males, patients on dialysis and medical staff. These groups were selected as they have a high HBV infection rate. Vaccine reduced the incidence of significant HBV infection in homosexual males, haemodialysis patients by 80-85%. HBsAg positive infection was not observed in vaccinated healthcare personnel.[28],[29],[30] The protective efficacy ranging from 89-100% was observed in infants born to HBsAg positive mothers. These studies have used either passive active immunoprophylaxis (HBIG at birth followed by a course of vaccine) or active immunoprophylaxis with HB vaccine alone. The protective efficacy was similar irrespective of study, vaccine brand, dose or schedule. Immunization with hepatitis-B vaccine with the first dose given at birth clearly prevents perinatal transmission of HBV infection in infants born to HBV carrier mothers.[26]

Vaccination schedule:

Two schedules are normally recommended for vaccination with hepatitis B vaccine. 0, 1 and 6 months and 0, 1, 2 and 12 months schedule. The latter may be used when rapid protection is required. 0, 1 and 6 months typically yields a high seroconversion rate and relatively high titers of anti-HBs that will persist for an extended period of time. The levels of antibody after 3 doses in 0, 1 and 2 months is lower hence a fourth dose at 12 months is needed to elicit a high titer of anti-HBs in these persons. Clinical trials have shown that subjects following 0, 1 and 2 and 0, 1 and 6 months schedule attained the seroconversion rates of 93% to 100%. The GMT is much higher in 0, 1 and 6 months schedule.[31] These schedules were selected to give maximum flexibility and optimum protection in different circumstances. The recommended doses (and sometimes the schedules) differ depending on the manufactures, the maternal HBsAg status and the presence of underlying disease.

A flexible schedule is needed to facilitate broad use of hepatitis-B vaccine. The local variations in the epidemiology of hepatitis B and the timing of other childhood vaccination make strict adherence to one or even two schedule for hepatitis-B vaccine impractical. The recommended schedules of vaccination depending on maternal status of HBsAg is shown in [Table:2].[32]

The Advisory committee on Immunization Practices (ACIP) considers both schedules equivalent. Both ACIP and The American Academy of Pediatrics (AAP) committees recommend that all children should receive three doses of vaccine by 18 months of age. The interval between first and second doses should be atleast 1 month, the interval between the second and third should be atleast 4 months. Because hepatitis-B vaccines are highly immunogenic in children when administered in variety of schedules pediatricians have successfully adopted alternative schedules to fit the needs of their practices.[32]

Special schedules:

For dialysis and other immunocompromised hosts a four dose regimen is recommended and vaccine is administered at 0, 1, 2 and 6 months. This is followed 1 month later with determination of anti-HBs titers. If the titers are below 10mIU/mL granulocyte macrophage colony stimulating factor (GMCSF) is given as an adjuvant along with a booster dose of 40µgms of hepatitis vaccine.[32]

Site of administration:

The site of vaccination and the method of administration of the vaccine are critical factors in achieving maximal seroconversion rates. The vaccine should be given intramuscularly. In adults the injection should be given in the deltoid region and in neonates and infants the vaccine should be given in anterolateral thigh region because of the small region of deltoid muscle. Persons vaccinated in gluteal regions have lower seroconversion rates and antibody titers.[13],[20]

In exceptional cases as the patients with severe bleeding tendencies such as Hemophiliacs the vaccine can be administered subcutaneously. The vaccine should never be given intravenously.[13] The high cost of recombinant vaccine has stimulated interest in the prospect of intradermal administration using a reduced dosage. A number of studies have been done to evaluate the immunogenicity of hepatitis-B vaccine administered intradermally. Most of the studies have found that both the seroconversion rate and GMT for anti-HBs to be significantly lower in adults given a reduced dosage of vaccine by intradermal route versus a standard dosage by intramuscular route.[33] Further intradermal vaccination is a much more difficult technique and is associated with higher incidence of local reactions including necrosis.

Dosage of Vaccine:

Human immunogenicity is dependent on the tertiary and quaternary structure of HBsAg polypeptide particles. These Properties may differ somewhat between the products. Thus the recommended dosage of the vaccine differs. The recommended dosage for the two vaccines are shown in [Table:3]. Clinical studies led to a selection of 40µgms dosages of HBsAg for patients with end stage renal disease receiving haemodialysis. Even with this regimen seroconversion rates are suboptimal ranging as low as 32% in elderly patients and upto 86% in young adults.[23]

Who should be vaccinated?

The initial recommendation strategies of ACIP have been selective vaccination of persons identified with high risk factors. The high-risk groups were identified as follow:

1. Health care workers have been a primary objective since vaccine became available in 1982. HBV infection is a well-recognized occupational hazard among hospital staff. Medical and paramedical staff whose daily practice involve patient care and frequent contacts with blood, blood products and body fluids require vaccination. Vaccination should be completed for students of the health care professions before they have blood contact.

2. Patients with chronic renal failure prior to dialysis treatment.

3. Recipients of blood &certain blood products eg. Haemophiliacs, Thalassemia.

4. Household contacts and sexual contacts of HBV carriers.

5. Infants born to HBsAg positive mothers must receive appropriate prophylaxis.

6. Sexually promiscuous group.

7. Parenteral drug abusers.

8. Travelers to endemic regions

9. Inmates and staff of institutions for mentally handicapped.

10. Vaccination to adolescents can be implemented to communities where drug abuse, pregnancy among teenage are common.

When studies demonstrated the post exposure efficacy of hepatitis-B vaccine in preventing infection in infants born to HBV infected mothers, additional recommendations were introduced such as screening of HBsAg in pregnant women to identify infants requiring prophylaxis. Later other studies showed that selective screening of women in high-risk groups failed to recognize many infected women. Therefore universal screening of all pregnant women was recommended in 1988.[32],[34] This strategy has been shown to be cost effective compared with other routine prenatal and neonatal screening programs.

Given the failure of the targeted high risk immunization strategy, the committee on Infectious Diseases, The American Academy of Pediatrics (AAP) and ACIP published the revised recommendations of HBV control including the recommendations for universal immunization to achieve greater control of HBV infection.[35],[36],[37]

Universal Immunization:

Elimination of HBV transmission will require a universal immunization policy with vaccination of more than 90% of the population. This goal can be achieved only in infancy, and infant immunization is the most cost-effective option. The vaccination of infants is more cost effective than vaccination at an older age because the dosage of vaccine for infants is lower than for adolescents and adults. The compliance is better in infants if the vaccine is integrated in EPI. But the full impact of universal infant immunization will be observed only after few decades therefore the committees recognizes the need for continuing to target high risk older children and adults for immunization.[35],[36]

The various factors influencing decision to recommend universal immunization of infants with hepatitis-B vaccine are shown in the [Table:4].[36]

Current Recommendations for Vaccination against HBV infection:

The AAP and ACIP currently recommend a combined strategy that is shown in [Table:5].[35],[36],[37]

In 1993 WHO has recommended that, HBV vaccination be introduced in the expanded program of immunization (EPI) of all countries with a high prevalence rate of HBV by 1995. The remaining countries should adapt to this programme by 1997.[32],[37] Introduction and implementation of universal vaccination has shown a significant reduction in HBsAg carrier rate among children in Taiwan. Ten years of this program has reduced the carrier rate in children from 10% to less than 1%.[38] In Thailand HB vaccine was given simultaneously with other EPI vaccines which has resulted in a significant reduction of carrier rate (6% to 0.23%). They are expecting virtual eradication of HBV infection.[39]

Options for India:

The current recommendation from ACIP and AAP is universal immunization of infants. The policy of universal immunization in infants has been found to be cost effective in our country. In fact in the long term, it has been calculated to cost less per case prevented, than a policy of screening and vaccinating only high risk newborns.[40] The adoption of HBV vaccination in the EPI by 1995 was first recommended by Indian Academy of Pediatrics (IAP) which was followed by resolutions from Indian Society of Gastroenterology and Indian Association for study of Liver (INASL).

There are several practical problems associated with the introduction of universal HBV vaccination in our country. Firstly the high cost of vaccine has hampered the inclusion of this vaccine in EPI. Till recently the vaccine needed to be imported which adds to the exchequer. Now with the availability of indigenous vaccine the cost of the vaccine has come down which may be affordable for mass use. The universal immunization at birth is not feasible in Indian context, given that only a small proportion of births occur in hospitals and others are inaccessible. To individual families who can afford the vaccine the option of immunization should be offered.

 Other Issues Regarding Hepatitis B Vaccination



Immunization of premature infants:

It is safe to immunize healthy premature infants at birth or before discharge from hospital. Although the antibody response may be slightly lower in premature infants than in term infants, most premature infants respond well to the vaccine.

Need for subsequent booster dose:

Adult recipients of plasma derived HBV vaccines have been protected from acute and chronic illness for as long as 9 years. 20-60% of vaccinated adults have titers more than 10mIU/mL 5-9 years of post immunization, immunologic memory is retained and anamnestic antibody response occurs on exposure to HBV. The immunologic memory is maintained for atleast 10 years.[41] The full duration of protection induced by three doses of HBV vaccine administered during infancy has not been determined, but protection for at-least 5 years is reported for infants of carrier mothers, who were immunized during infancy.[42] Currently after the initial three vaccines doses neither serologic testing nor any further booster doses are recommended. Booster dose is recommended for children born to HBsAg and HBeAg positive mothers but not for others.[42],[43] Health care workers because of their high risk should be screened for anti-HBs every 2 years and if anti-HBs levels fall below 10 mIU/mL they should be given booster and response monitored after 4 weeks. Patients with impaired immune function tend to have lower peak of anti-HBs levels. Hence, routine booster vaccination is recommended for such patients after testing for anti-HBs every 6-12 months. Because of conflicting data on use of booster in healthy adults no recommendations can be given at present. Economic feasibility can be the deciding factor.[43]

Prevaccination and post vaccination testing for antibody:

Whether or not to test vaccine candidates for susceptibility is essentially an issue of cost effectiveness. Prevaccination screening is appropriate in populations with high prevalence of HBV markers in many groups such as haemodialysis patients, household contacts of carriers, hospital staff, patients with frequent blood requirements, male homosexuals, IV drug abusers. While it is typically not cost effective in low prevalence populations such as health professionals in training or the routine immunization of children, most adolescents and mass vaccination. The ACIP does recommend post vaccination antibody testing of persons whose management depend on knowing their immune status (dialysis patients and staff, infants born to HBsAg positive mothers and persons with HIV infection). Post vaccination testing should also be considered for persons at occupational risk who may have needle stick exposures necessitating post exposure immunoprophylaxis. When necessary the test for anti-HBs should generally be done 1-6 months after completion of primary vaccine series. Infants born to carrier mothers should be tested 3-9 months after completion of vaccination. In the general population, decisions should be taken depending on other factors and economic feasibility. Prevaccination screening is not cost effective in India in most groups (except high risk). Thus it cannot be recommended routinely but there are ethical issues involved and to that extent it should be an individual's educated decisions.[43]

Should pregnant females be vaccinated?

AAP recommends vaccination of pregnant women at high risk for HBV infection. There are no reports on any adverse effect to the fetus in a mother who has been vaccinated during pregnancy. However vaccination during pregnancy is a matter of clinical judgement and is reasonable to delay vaccination until the second trimester. Vaccination depends upon relative risk and benefits. If the risk is justified, vaccine is administered at 0,1,2 months followed by a booster at 12 months. Women need not be tested for pregnancy before vaccination.[32]

Revaccination of nonresponders and / or low responders:

Non responsiveness is defined as the failure to acquire detectable levels of anti-HBs after vaccination. Hyporesponsivness is defined as failure to acquire protective levels of anti-HBs (>10 mIU/mL) following vaccination. Persons who do not respond to the primary vaccine series are revaccinated with one or two additional doses of hepatitis-B vaccine. About 15-25% of them produce adequate antibody response after one additional doses and 30-50% after 3 additional doses. Non responders should be counselled carefully on safe work practice to avoid infection. In hyporesponders revaccination is successful in inducing immune response in most patients. The response is more likely to be adequate and sustained.[1],[3],[32]

Improving the immune response:

Considerable attention has been and is being paid to increase the immune response to hepatitis-B vaccine, especially in genetic non-responders, in the immunocompromised such as renal dialysis patients and in the elderly.

One concept being explored is that of inclusion of polypeptide of Pre S1 and Pre S2 regions in the vaccine that might increase the epitope repertoire, especially for T helper epitopes. The pre S domains are immunogenic and elicit antibody responses during natural HBV infection. The anti Pre S antibodies often appear prior to any other HBV specific antibody. The contribution of the Pre S domains to a protective immune response in humans is not resolved, but there is evidence to indicate that they may be suitable vaccine components.[44]

Adjuvants to increase vaccine potency:

There are a number of at risk population such as patients receiving hemodialysis, organ transplant recipients, persons infected with HIV, oncology patients who respond poorly to existing vaccines and need more potent vaccines or vaccination regimens. Use of immuno stimulating lymphokines or other immuno modulators as adjuvants has shown to be beneficial in small controlled studies on patients receiving hemodialysis. Thymopentin and interleukin 2, recombinant gamma-interferon and GMCSF are some of the immunomodulators which have shown to improve response to HBV vaccine. These have been used in varying dosages and regimens.[44]

HBV Mutants:

HBV virus although a DNA Virus, passes through a RNA stage in its replication in which mutations may occur. Two types of HBV mutants are known so far 1) Core gene mutation 2) 'S' Gene mutations.

Mutation of 'S' gene (envelope variants):

The currently licensed hepatitis B vaccine contain the major surface protein and an immune response to the immunodominant 'a' epitope appears to induce protection to reinfection. This epitope which covers amino acid residues 124-145 is common to all subtypes of the virus and appears to have a double loop conformation [Figure:3]. A Glycine Arginine substitution in the common 'a' determinant of 'S' gene occurs between amino acids 142-145. This results in the formation of an envelope protein, which is undetectable by conventional EIA. Alternatively HBsAg is not at all synthesized and naked DNA particles circulate. Three types of surface mutations are known.

1) Vaccine escape mutants: These are seen in patients who have generally received vaccination against hepatitis - B. These patients contract fresh HBV infection despite levels of adequate anti-HBs and can develop cirrhosis and HCC. This was first detected in children born to HBsAg carrier mothers in Southern Italy.[45]

2) Immunoglobulin escape mutants: Patients of chronic Hepatitis B who undergo transplantation and are subsequently treated with high titers of HBIG can have resurgence in their HBV infection even in the presence of high titers of HBIG. Point mutation at 145 A occurs under immune pressure. Chronic infection by this mutant leads to loss of graft tissue.[46]

3) Naturally occurring surface mutants. There are naturally occurring mutants of 'S' gene, which occur during chronic infection. These have been isolated from India.47

Thus there are evidences pointing to sucespitibility to mutation of 'S' region under “immune pressure”. Long term follow up studies and surveillance are clearly required. Consideration may have to be given to the design of future hepatitis-B vaccines to prevent emergence of this escape mutant.

Multivalent childhood vaccines:

At present hepatitis B vaccines are available only in a monovalent formulation. An addition of this vaccine to the standard set of pediatric immunizations thus requires the administration of an extra injection to a child on at least three occasions. Acceptance of routine childhood immunization against hepatitis B would be enhanced if HBsAg could be included in polyvalent formulations with other pediatric vaccines. Since HB vaccine is an inactivated vaccine given in multiple doses, combination with the DTP vaccine is considered an option to explore. A study was performed with Smith Kline Beecham Biologicals DTP-HB vaccine. The results showed that the reactogenicity of the combined vaccines were comparable to DTP vaccines in terms of local swelling and general irritability but the frequency of fever was more in combined group. No serious adverse events were reported. The combined DTP-HB vaccines were able to elicit satisfactory antibodies to all 4 antigens. Follow-up until 6 months showed that all antibodies were in sufficient levels and may be long lasting.[27]

 Future considerations:



Currently licensed recombinant hepatitis B vaccines have established a remarkable safety and efficacy record. Efforts should now be concentrated on a wider application of these vaccines and large immunization campaigns aimed at newborns and children. Development of new vaccine may provide immunogens to overcome nonresposivness to HBsAg.[48],[49] Efforts have been mainly concentrated on the analysis of the Pre-S domain to protective immunity. Results on the Pre-S2 domain have been disappointing as the potency of pre-S2 containing recombinant vaccines was generally lower than that of currently available HBs(S) vaccines, and occasional cases of non responsiveness to vaccination were still found. Inclusion of T-helper epitopes from the core has been proposed as yet another way to reduce the percentage of non-responders.[49] Development of combination vaccines with DTP, Hib conjugate and possibly enhanced inactivated polio or hepatitis A vaccines are also considered. In the immunocompromised non-responders a new direction is the use of immunomodulators in conjunction with existing vaccine to induce improved immune response. Another area of hepatitis B vaccine research includes studies on long term protection and the need for a booster dose at a later age when the vaccine is given in infancy. In the long term there is a possibility that oral vaccines capable of inducing a long lasting protective systemic immunity may be developed.

 Acknowledgement



We gratefully acknowledge the secretarial assistance of Mr. B.Sreenivasulu, Jr.Laboratory Investigator, Department of Gastroenterology, in preparation of the manuscript.

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