[Irenic]

In 1947 Salk became head of the Virus Research Lab at the University of Pittsburgh. He began investigating the poliovirus. To start with, he had to sort the 125 strains of the virus. He found that they fell into three basic types and knew that a vaccine would have to include these three types to protect against all polio. One of the hardest things about working with poliovirus was manufacturing enough to experiment withÑand to make vaccine production practical.
http://www.pbs.org/wgbh/aso/databank/entries/dm52sa.html

Are there new strains?

[Tilted Irish Kilt]

It should be noted that Salk did not patent his vaccine.
He gave it for " the greater good of humanity " ,
rather than patent it and make millions of dollars in royalties .