Polio vaccine incidents reduced public confidence in polio vaccine leading to a drop in vaccination rates. At the same time that Salk was testing his vaccine, both Albert Sabin and Hilary Koprowski continued working on developing a vaccine using live virus. During a meeting in Stockholm to discuss polio vaccines in November 1955, Sabin presented results obtained on a group of 80 volunteers, while Koprowski read a paper detailing the findings of a trial enrolling 150 people. Sabin and Koprowski both eventually succeeded in developing vaccines. Because of the commitment to the Salk vaccine in America, Sabin and Koprowski both did their
Polio vaccine attenuate the Sabin 2 strain, and 10 substitutions are involved in attenuating the Sabin 3 strain. The primary attenuating factor common to all three Sabin vaccines is a mutation located in the virus's internal ribosome entry site, which alters stem-loop structures, and reduces the ability of poliovirus to translate its RNA template within the host cell. The attenuated poliovirus in the Sabin vaccine replicates very efficiently in the gut, the primary site of infection and replication, but is unable to replicate efficiently within nervous system tissue. In 1961, type 1 and 2 monovalent oral poliovirus vaccine (MOPV) was licensed, and