We have compared the immunogenicity profile of a recombinant lipoprotein outer-surface protein A (OspA) Lyme disease vaccine administered on schedules of 0, 1, and 6 months (group 1) or 0, 1, and 12 months (group 2) to 800 healthy subjects, aged 15-50 years. One month after the second dosing, geometric mean titers of IgG antibodies to OspA were 1,309 ELISA units (EL.U)/mL in group 1 and 1,404 EL.U/mL in group 2. One month after the third dosing, the titers were 7,205 EL.U/mL and 10,659 EL.U/mL, respectively. Using bioequivalence methodology, we showed that the two vaccination schedules elicit an equivalent immune response 1 month after administration of dose 3: at that point, 91%-93% of all subjects had titers > or =1,400 EL.U/mL, proposed to be protective for one tick season. The vast majority of local and systemic symptoms were mild to moderate and of limited duration. The 0, 1, and 6 months vaccination schedule is a viable alternative to the 0, 1, and 12 months schedule and can provide protection against Lyme disease during one tick season.