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Vaccine-Preventable Diseases
Hepatitis B
Hepatitis B virus (HBV) is one of several viruses that cause hepatitis. HBV is a double-stranded DNA virus with three major antigens, known as hepatitis B surface antigen (HBsAg), hepatitis B e antigen (HBeAg) and hepatitis B core antigen (HBcAg). HBsAg can be detected in serum 30 to 60 days after exposure and persists until the infection resolves. Any person positive for HBsAg is considered infectious. In most cases, anti-HBs appears after HBsAg has disappeared and the infection has resolved. In severe acute HBV infections, anti-HBs may be present simultaneously with HBsAg. In a proportion that varies inversely with age, infection persists. Anti-HBs confers long-term immunity.
HBcAg never appears in serum. Anti-HBc develops in all HBV infections, is not protective and persists indefinitely as a serologic marker, both in chronic active infection and in resolved infection after clearance of HBsAg. Anti-HBc IgM is a marker of recent HBV infection. It appears during the first week of acute hepatitis illness and is usually present for 6-12 months. It can be used to diagnose recent acute hepatitis B. In 10%-15% of cases of chronic hepatitis B infection, IgM anti-HBc may also be detected, particularly when a replication HBV is present. HBeAg is associated with viral replication and high infectiousness. Anti-HBe usually indicates a reduction in replicating virus and lower infectiousness. Methods of quantification of HBV DNA in serum are available to assist in determining both infectiousness and prognosis.
Initial infection with HBV may be asymptomatic in up to 50% of adults and 90% of children. The incubation period is 45 to 160 days, with an average of 120 days. When symptoms occur, they include an insidious onset of anorexia, vague abdominal pain, nausea, vomiting and jaundice. Acute illness may last up to 3 months and has a case fatality rate of 1% to 2%, which increases with age. Fulminant hepatitis and death may also occur in pregnant women and in infants born to infected mothers.
An individual with either acute symptomatic or asymptomatic HBV infection may become a chronic carrier. A chronic carrier is an individual from whom serum samples taken 6 months apart are HBsAg positive or a single serum sample is HBsAg positive and anti-HBc IgM negative. The risk of becoming a chronic carrier varies inversely with the age at which infection occurs (infants: 90% to 95%; children < 5 years: 25% to 50%; adults: 3% to 10%). The risk of becoming a chronic carrier is also greater in immuno-compromised patients. Chronic carriers often do not have overt disease but over time are at increased risk of developing hepatic cirrhosis and primary hepatocellular carcinoma. All carriers should be considered infectious.
Epidemiology
The epidemiology of the disease has been considerably modified by the introduction of the universal program of immunization and the increased use of vaccine in targeted groups. The incidence of hepatitis B has been decreasing in all age groups in recent years, coinciding with the increasing use of the vaccine (see Figure 5). HBV infection has virtually disappeared in the cohorts that have benefited from the universal immunization programs.
HBV is found mainly in the blood, vaginal secretions, semen and serous fluids of an infected person. It is present in the saliva at concentrations 1,000-10,000 times less than in blood. HBV is transmitted through percutaneous or mucosal contact with infectious biological fluids. It is transmitted from infected mothers to newborns and in settings of close personal contact through unrecognized contact with infectious bodily fluids. It is transmitted through sexual contact, both heterosexual and homosexual, and through contact with blood (needle stick, intravenous drug use with needle sharing). The risk of transfusion-related hepatitis B is extremely low because of routine HBsAg and anti-HBc screening of donated blood and exclusion of donors at risk of infection. The precise role of saliva in the transmission of HBV is not clearly known. Saliva is considered infectious in bite wounds with broken skin involving the percutaneous inoculation of saliva, or when it is visibly tainted with blood. In Canada, like elsewhere, almost one-third of infections have no identified risk factors, despite thorough enquiries.
The regions of the world with the highest prevalence of infection are South-East Asia and Africa, but the use of vaccine in some of these countries for the last 20 years has reduced drastically the incidence of hepatitis B. Although there are no national data on the prevalence of chronic HBV infection for the whole Canadian population, Canada is considered an area of low endemicity. It is estimated that < 5% of residents have markers of past infection, and < 1% are HBsAg carriers. This will vary in different subgroups of the population according to the presence of the factors listed earlier and the vaccine coverage achieved.
Source: Canadian Immunization Guide, 7th edition, 2006
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Guidelines and Recommendations
- For information about Hepatitis B vaccine, see the Canadian Immunization Guide, 7th edition 2006
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