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Articles : Hepatitis B immunization in current clinical practice
Hepatitis B vaccine was first made available in 1982. The initial plasma derived vaccine was improved over the years and the current hepatitis B vaccines are manufactured using recombinant DNA technology and contains a portion of non-infectious hepatitis B virus gene coding for the HbsAg. Malaysia started routine Hepatitis B immunization for infants in 1989 using the three-dose regime with the first dose given shortly after birth, thereafter at 1 month and then at 5 months old to coincide with the third dose of DTP/OPV. Anti-HBs immunoglobulin is also given together with the first dose of hepatitis B vaccine (active-passive immunization) at different sites if the mother is found to be a carrier of Hepatitis B virus. Hepatitis B immunization prevents both vertical and horizontal transmission. The immunogenicity and efficacy of current hepatitis B vaccines in preventing newborn babies from becoming infected and subsequently becoming chronic carriers of the hepatitis B virus (HBV) had long been established through many studies. This is important because a chronic carrier state is associated with the development of hepatocellular carcinoma. Studies conducted by the Malaysian Liver Foundation have found that 80% of the adults diagnosed to have hepatocellular carcinoma are chronic HBV carriers. Indeed, the hepatitis B vaccine is credited as being the first anti-cancer vaccine to be developed. Medical practitioners often face a dilemma over three practical issues, which this short article will try to address. The emphasis is to give a perspective of what is already known about the current hepatitis B vaccine and its use in protection against HBV infection so that practitioners can make an informed decision on advising their patients. Knowledge on immunization against hepatitis B and its duration of protection is still ongoing. Hence, the practice of hepatitis B immunization may change in the near future as new knowledge emerges. Medical practitioners therefore will need to keep abreast of such knowledge so that their practice can be current. Pre-vaccination serologic testing The need to have blood tested to document the presence of protective anti-HBs antibody is to identify those who do not need to be vaccinated unnecessarily and hence save cost on purchasing the vaccine. In the majority of cases in healthy infants, children and adults this is unnecessary and should be guided by the likelihood that the individual had been exposed to HBV and/or another family member had been found to be infected or a carrier of the HBV. The administration of the vaccine in an immune or infected individual will not result in any adverse outcome. In the event that pre-serologic testing is deemed necessary, a combination of tests for HBsAg, anti-HBsAb and anti-HBcAb will differentiate those who are currently infected or had past infection (detectable anti-HBc antibody) from a carrier state (HBsAg positive) who can then be followed up and treated. Post-vaccination serologic testing The current hepatitis B vaccines are highly immunogenic with > 95% of children and adolescents developing protective antibody following the recommended three doses of vaccines. Thus, routine post-immunisation serologic testing is not recommended except in the following situations:
Post-vaccination serologic testing is usually carried out 1-3 months following the last dose of the vaccine. Anti-HBs antibody levels of = 10 mIU/L are considered protective against infection. Since low birth weight babies (< 2000 gm) respond poorly to the vaccine 1-3, one might even consider extending post-immunisation serologic testing in such situation. In a recent publication by WHO 4 it is recommended that this group of babies be given four doses of the vaccine, with the first dose given shortly after birth not considered as part of the primary series. To boost or not to boost ? Following the recommended three doses of vaccines the duration of immunity lasts at least 13-15 years 5 and seems to be longer in those with a higher post-immunisation antibody titre. The decline in the antibody titre is most rapid in the first year, after which the rate of decline slows down considerably. However, a low or undetectable anti-HBs Ab does not mean that there is loss of protective antibody 6. Nevertheless, this observation of waning antibody level to < 10mIU/mL has generated some anxiety among practitioners and unwittingly brought about the controversial subject of whether a booster (fourth) dose of the vaccine is needed in 5-10 years after the last dose of the vaccine. In those subjects who have responded (anti-HBs antibody titre equal to or more than 10 mIU/mL) the duration of immunity is at least 15 years. Since the current hepatitis B vaccines are highly immunogenic in over 95% of immunocompetent vaccinees the large majority of children will retain long-term protection, unless there are reasons to believe otherwise. Protection against HBV is conferred by memory T-cells and B-cells, in those successfully vaccinated regardless of persistence of detectable antibody. This anamnestic response occurs on exposure to the HBV either naturally or by booster immunization. 7-12 Based on these observations regular booster doses of hepatitis B vaccine are not indicated for immunocompetent vaccinees. Indeed, currently none of the advisory bodies (Advisory Committee for Immunization Practices (ACIP) in the United States nor the European Consensus Group 13 on Hepatitis B Immunity) recommend a routine booster dose of hepatitis B vaccine following the primary series of three doses, regardless of whether or not the first dose was administered shortly after birth. Summary :
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Prepared by Dr MT Koh 5 Dec 2004 Modified and corrected : April 2005 |
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