Hepatitis B is spread by blood and secretions. Although those receiving blood transfusions are at risk, screening of blood with hepatitis B surface antigen (BsAg), hepatitis B core antibody (BcAb) and nucleic acid testing (NAT) has made post-transfusion HBV rare. Recommendations for adult HBVimmunization focus on those at high risk due to blood or sexual exposure.

Blood exposure groups include healthcare workers, hemodialysis patients and intravenous drug users. Routine immunization has made HBV rare in the former two groups. When HBV does occur among healthcare workers or hemodialysis patients, it is related to primary vaccine non-response or secondary loss of vaccine effectiveness due to host immunosuppression.

In the Centers for Disease Control and Prevention Sentinel County project, 5.6% of those with acute hepatitis B had been incarcerated during theincubation period for HBV. This points to the central role of high-risk activity and also the potential benefit of vaccinating inmates.

Sexual spread is the most common risk factor for the spread of hepatitis B. The greater the number of sexual partners, the more the risk.

HBV Vaccine Efficacy
Hepatitis B vaccine induces immunologic memory as a function of memory B cells. The first dose of HBV vaccine initiates clonal expansion of memory B cells and, once initiated, this memory continues even after BsAg and even B surface antibody (HbsAb) are longer detectable. The third dose induces a secondary immunologic response that increases HBsAb titers which are the neutralizing antibodies providing protection against HBV infection.

Clinical trials involving adults have shown that a single dose of HBV vaccine induces memory B cells.A second HBV vaccine dose four years later induces a prompt rise in HBsAb. If, after developing seroprotection, antibody titers wane, re-exposure to either HBV vaccine or hepatitis B infection is followed by an exponential HBsAb rise within three to five days. This is fast enough to prevent HBV that has entered the body from invading hepatocytes in sufficient quantities to cause clinical infection.That a second (or third) dose given years later induces a vigorous immunologic response is the reason that, if a hepatitis B vaccination has a long interval between the first and subsequent doses, the series is resumed, not restarted.