<qa>  What is the interval between the blood sample and the hepatitis B vaccine dose received- Documented response to previous hepatitis B vaccination i.e. anti-HBs antibody concentrations 3 10 mIUml after primary vaccination or after boosters with licensed vaccine, but for whom there is a loss of anti-HBs antibody concentrations below 10 mIUml at the time of inclusion into the study. Patients who have antibody concentrations below 50 mIUml at the time of inclusion will also be recruited provided that this antibody concentration is less than half of the highest documented antibody response achieved after primary vaccination or boosters. The interval between the blood sample corresponding to the documented response and the hepatitis B vaccine dose received prior to this blood sample should be at least 25 days 