http://www.healthline.com/
An excerpt:
Luft told Healthline that the key to creating the vaccine was to bioengineer a protein not found in nature. Using the scaffolding of a protein known as OspA, which is found on Borrelia's outer surface when it lives in ticks, the scientists created a new fusion protein called a chimera. The chimera contains proteins from a wide range of Borrelia bacteria, making the vaccine effective against many different strains.
Remember how Jurassic Park ended? "Nature" always find a way. Especially in the case of the highly complex, adaptable Borrelia genome.