HBV has acute and chronic phases of infection; the majority of infected patients have asymptomatic subclinical infection, but up to 30 per cent experience symptomatic acute hepatitis, and 0.5–1 per cent will develop fulminant hepatic failure.
Ninety to 95 per cent of infected patients recover completely, but between 2 and 10 per cent of those infected as adults become chronic carriers, indicated by the persistence of HB surface antigen (HBsAg) for more than six months.
Approximately 25–40 per cent of patients with chronic hepatitis B die from cirrhosis or liver cancer, and more than 300,000 cases of HBV-related cancer occur each year, making it the second most common carcinogen after tobacco.
The virus may be transmitted by contact with infected blood or body fluids. Transmission mostly occurs sexually, parenterally or by vertical transmission from an infectious mother to her unborn child.
The prevalence and pattern of HBV transmission varies greatly between different populations and different areas. The world can be divided into three categories based on the prevalence of chronic HBV infection.
In high prevalence areas, most infections are acquired at birth or during early childhood when the risk of developing chronic infection is greatest.
In moderate prevalence areas, many infections occur in adolescents and adults, however, the high rates of chronic infection are maintained mostly by infections occurring in infants and children.
In low prevalence areas, most HBV infections occur in adults in relatively well-defined risk groups.
Risk for travellers
The risk of HBV infection for travellers is low, with estimated incidence of one case in 2,000–10,000 per month in short-term travellers, and one case in 1,000 per month for expatriates.
For travellers in high-endemicity areas, respective incidence rates of symptomatic and asymptomatic infections are 60 and 360 per 100,000 per month in Asia, and 20 and 60 per 100,000 per month in Africa and Latin America.
Factors to consider in assessing the risk to travellers include the prevalence of chronic HBV infection in the local population, the likelihood of direct contact with blood or secretions, or of sexual contact with potentially infected people and the duration of travel.
The HBV vaccine was licensed over 20 years ago and, in the UK, pre-exposure immunisation is offered to individuals who are at increased risk of acquiring HBV.
In the UK, three schedules are generally used. The rapid schedule consists of doses at 0, 7 and 21 days (licensed for Ž18 years). A fourth dose should be administered at 12 months if still at risk.
The accelerated schedule consists of doses at 0, 1 and 2 months. A fourth dose should be administered at 12 months if still at risk.
The basic schedule consists of doses at 0, one and 6 months. The three-dose series is generally associated with high efficacy and an excellent safety record. The vaccines induce antibodies that will neutralise HBsAg (anti-HBs), and historically, the immune response post-vaccination has been assessed by measuring anti-HBs. Although anti-HBs levels above 10mIU/ml are generally taken to be protective, the potential for low anti-HBs levels to mask significant infection has led some countries to adopt a higher reference level (for example 100mIU/ml in the UK).
Although vaccination against HBV is successful, 10 to 15 per cent of adults fail to respond.
The most common patient-related factors for the failure of the vaccine include older age, male sex, history of smoking and obesity.
Altering the dose schedule and route of vaccination and using adjuvants may enhance responses in patients.
Generally, testing for markers of past or current HBV infection is not indicated before immunisation. It can be considered when vaccinating those from groups with high rates of HBV infection, but if testing is indicated, vaccination should not be delayed.
Under the Control of Substances Hazardous to Health Regulations, those at occupational risk have the right to know whether or not they are protected.
To boost or not to boost?
The full duration of protection afforded by the hepatitis B vaccine has yet to be established, but studies suggest that it is effective in protecting against development of clinical disease and chronic infection for at least 15 years. It also mitigates the clinical effects of subsequent infections.
Anti-HBs levels decline with time elapsed since the first vaccination, to below 10mIU/ml in up to 50 per cent of vaccinated patients at 10 years, but vaccinated responders have persisting anti-HBs and cellular immunity and can mount a rapid and powerful anamnestic response to a vaccine challenge.
In the UK there is concern that not all patients respond in this way, and it is recommended that patients at continuing risk of infection should be offered a single booster dose of vaccine, around five years after primary immunisation.
After a primary course of vaccine, in immunocompetent travellers, there is generally no need to test for anti-HBs levels post-vaccination, and a booster dose of vaccine is not required unless the traveller remains at risk.
Dr Patel is an occupational health physician and director of clinical services at MASTA, London
- Offer to travellers to areas of intermediate or high prevalence who place themselves at risk.
- Pre-vaccination — not generally indicated but consider in groups with high rates of HBV infection.
- Post-vaccination — again not generally recommended but consider for certain groups where knowledge of immune status is essential for their medical management (for example immunocompromised or those at occupational risk).
- There is evidence that immune memory can persist in those successfully immunised.
- Not all individuals may respond in this way, so offer a single booster to those at continued risk.