However, despite low anti-HBs concentrations, HBV infections (as measured by the presence of markers of infection such as hepatitis B surface antigen [HBsAg] or antibody to hepatitis B core antigen [anti-HBc]) are rare in persons who have been shown to respond to the primary vaccine series. Continued protection against HBV infection in these children is likely provided by immune memory cells generated at the time of the primary series. An immunologic memory response (also termed an anamnestic response) can be evaluated by measuring the effect of an additional (booster) dose of hepatitis B vaccine. An anamnestic response is generally defined as a rapid (within 14-30 days) two-fold or greater increase in the anti-HBs titer to at least 20mIU/mL. This signifies that immune memory cells capable of generating a rapid rise in anti-HBs are still functional and would protect against HBV infection. Blood tests that can identify memory B cells that are specific for vaccine antigens are now available, and could be used to provide additional evidence of immune memory.9