When and how often should someone get the hepatitis B vaccine? If someone has had hepatitis B in the past, should the person be vaccinated? How do we know if he has been cured?

N.K.

Hepatitis B is a viral infection of the liver that usually causes a self-limited infection with fever, pain in the right upper abdomen and jaundice (yellowing of the skin and eyes). Occasionally, the acute infection is severe and even fatal. In 5-10 percent of cases, however, the infection does not run its course, and chronic (persistent) infection develops, causing serious liver damage in some patients.

There is a vaccine to prevent hepatitis B. It is effective in preventing the infection in about 90 percent of cases. In patients who have a detectable response to the vaccine (high levels of disease-fighting proteins called antibodies), protection against hepatitis B infection is virtually 100 percent. Health authorities now recommend hepatitis B vaccine for all children in the United States, as well as for all adolescents and adults deemed to be at increased risk. It is very safe and there are few, if any, reasons not to be vaccinated. Hepatitis B vaccine is typically given in three doses over a six-month period.

Experts do not generally recommend testing every person to confirm response to the vaccine. But testing is probably a good idea for those at the highest risk of infection, to ensure that they are protected. Medical scientists don't how long the vaccine remains effective, but efficacy is likely to persist for at least nine years. There are no recommendations for booster injections (to "boost" the effectiveness of an immune response that may have diminished over time) in normal adults, though some experts recommend boosters for those with impaired immunity.

The vaccine is not given to someone who has had hepatitis B in the past, because such an individual has essentially no risk of acquiring hepatitis B again. A few simple blood tests can determine if a person has had hepatitis B, and if so, whether the infection was cleared from the body. Simply, if there is detectable antibody that "recognizes" a part of the hepatitis virus called surface antigen, then the person either had the vaccine or had hepatitis B infection and cleared the virus from the body. If the surface antigen itself is detected in the blood, then the patient has active infection, either acute or chronic. In some cases, where both these tests are negative in a patient who has had hepatitis B, sophisticated laboratory tests such as PCR (polymerase chain reaction) can usually indicate whether the infection was cleared.