Study design and major findings. Flowchart indicates number of subjects screened and included in the study. Thirty-six of 159 subjects had antibody to hepatitis B surface antigen (anti-HBs) levels <12 mIU/mL (group 1, 9/50 [18%]; group 2, 13/50 [26%]; and group 3, 14/59 [24%]). Thirty-four of 36 received a booster vaccine (group 1, 8/9 [88%]; group 2, 12/13 [92%]; and group 3, 14/14 [100%]). Thirty-two of 34 developed protective levels of anti-HBs 3 weeks after the booster vaccine (group 1, 7/8 [86%]; group 2, 11/12 [92%]; and group 3, 14/14 [100%]). Abbreviation: HBV, hepatitis B virus.