The first type was developed by Jonas Salk and first tested in 1952. Announced to the world by Salk on April 12, 1955, it is an injection of inactivated (dead) poliovirus. The second type was an oral vaccine developed by Albert Sabin, using attenuated poliovirus. An attenuated virus is a much less harmful strain of the same virus. Human trials of Sabin's vaccine began in 1957 and it was licensed in 1962. 