Mice vaccinated with recombinant outer surface protein A (OspA) have been shown to be protected from infection with Borrelia burgdorferi, the agent of Lyme disease, when sacrificed 14 days after challenge with an intradermal inoculum of the spirochete. To determine whether infection was not merely delayed and that protection was long-lasting, we sacrificed vaccinated mice 60, 120, and 180 days after challenge; and to determine whether vaccinated mice retained their immune state over long periods, we challenged mice with B. burgdorferi 60, 90, 120, and 150 days after vaccination. The results of both groups of experiments show that the mice remained free from infection and disease and extend the usefulness of OspA as a vaccine candidate for Lyme borreliosis.