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Subunit vaccines consisting of single recombinant outer surface proteins (Osp) of Borrelia burgdorferi have been highly successful in protecting mice against challenge by borrelial strains closely related to the strain from which the immunogen was derived. Humoral immunity is sufficient for protection. A dual mode of action for these vaccines has been suggested because serum factors ingested by the tick during the blood meal may begin to reduce the spirochete inoculum prior to transmission to the host. At present two different recombinant OspA vaccine preparations (monovalent) are being evaluated in humans in large-scale phase III efficacy trials in the United States. Local discomfort at the intramuscular injection site has been the principal adverse effect seen to date with these vaccines, but further data on safety are being collected. The greater heterogeneity of OspA among
Lyme Borrelia in Europe implies that a vaccine preparation containing multiple antigens (multivalent) may be necessary there, although this is also a concern in the United States.