Immunoprophylaxis is routinely recommended for infants born to mothers with hepatitis B virus (HBV) infection within the first 12-24 hours. Detection of hepatitis B surface antibody (HBsAb) resulting from hepatitis B immunoglobulin administered at birth may be perceived as a real vaccine response. This makes it difficult to detect HBV infection. For this reason, it is recommended that infants born to hepatitis B surface antigen positive mothers and who received immunoprophylaxis at birth should have HBsAb testing when they are 9-15 months old.