On April 12, 1955 Jonas Salk's polio vaccine was first licensed for public use in the U.S. In the years afterwards 90 million Americans were vaccinated in the largest mass vaccination campaign ever. Polio virtually disappeared from the continent and Jonas Salk became a hero. But not long after, 260 children who were vaccinated with Salk's vaccine became sick. Eleven of them died. An investigation showed that some lots of the vaccine were defective and confidence in it was shaken. Two scientific greats, Dr. Hilary Koprowski, director of the Wistar Institute in Philadelphia, and Dr. Albert Sabin, a physician at the Cincinnati Children's Hospital, raced to replace the Salk vaccine. Dr. Cecil Fox, a pathologist at the National Institute of Infectious Diseases remembers that it was an intense competition. Both Sabin and Koprowski's vaccines were derived from monkey organs. They needed to test them in large, non-immunized population groups, no longer available in North America. Sabin traveled to the U.S.S.R. and vaccinated more than 6 million people in Latvia, Estonia and Kazakhstan between 1958 and 1959. No AIDS cases emerged in the U.S.S.R. where Sabin did his testing. Koprowski moved his study to the Belgian Congo which had one of Africa's more modern health care infrastructures at the time. Between 1956 and 1960 more then 1 million African people were 'encouraged' to receive Koprowski's vaccine called CHAT. Sabin analyzed Koprowski's vaccine in 1958 and found it be be 'unstable and contaminated by an unknown virus'. He told Koprowski about his discovery and went then went public with his findings.