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OspA Heterogeneity

While it has been observed that OspA varies greatly or is completely

absent in European isolates, it was hoped that OspA vaccination would provide

protection for vaccinated persons in North America where there is less OspA

heterogeneity.[19] Recently, however, a growing concern about diversity among

North American isolates has been noted.[20-31] The ability of some OspA

serotypes to avoid killing with antibodies raised against other serotypes has

been shown.[24,31] In the study by Lovrich et al,[27] the authors concluded that

although cross protection occurred against some strains expressing different

antigenic types of OspA, vaccination with a single OspA type did not provide

complete protection against challenge with all strains. Even more surprising was

the finding that the presence of anti-OspA antibodies elicited from some

isolates did not result in protection against challenge with the homologous

strain.

The current OspA vaccine utilizes a Borrelia burgdorferi sensu stricto

OspA molecule which, to date, has been found in the majority of the isolates

from North America.[19,26] However, one type of North American OspA variant,

typified by strain 250[15], has been shown to infect mice vaccinated with N40

OspA,[21] a molecule similar to the current OspA vaccinogen. This variant type,

isolated from upstate New York, has also been isolated from Illinois,[32]

(presented by Picken, 11th Annual Scientific Conference on Lyme

Borreliosis, New York, April 25-27, 1998). It is therefore possible that,

individuals infected with this variant strain may not be protected with the

current vaccine. Furthermore, the probability of discovering other variants

against which the vaccine will fail is high given the propensity for the

organism to undergo mutational and recombinational events at the OspA

locus,[33-38] and the discovery of variant Borrelia strains in locations such as

California,[25,39] New York,[30,40] Texas,[40] Missouri,[40,41] Illinois,[32]

Georgia, and Florida,[42-44] which have yet to be tested in vaccination

protocols.

While the greatest variation of the OspA molecule occurs in European

isolates, the increasing evidence of OspA variability in North America, together

with the observation that cross protection is not always achieved with OspA

vaccination, implies that even a vaccine that includes several serotypes of OspA

molecules will not result in complete protection of the vaccinated North America

population.