This laboratory-based substudy of ATN 024 and 025 will evaluate the genetic contribution to highly individualized immune responses to hepatitis B vaccine in individuals and confirm the correlation of specific human leukocyte antigen (HLA) alleles and haplotypes with Hepatitis B Virus (HVB) antibody concentrations and antibody decay kinetics in vaccinated adolescents. Approximately 5 ml of whole blood will be collected from study participants at the time of the week 28 visit or at any subsequent study visit or clinic visit following successful completion of the week 28 visit. Peripheral blood mononuclear cells will be obtained and QIA amp Blood kit will be used to extract high-quality genomic DNA for polymerase chain reaction-based genotyping by the PEII laboratory.