The bitter rivalry between the developers of the original polio vaccines, Salk and Sabin, was a contributory factor to the United States and the Netherlands choosing to pursue different polio vaccination programs. In April 1955, the results of the largest clinical trial ever held (at that time) were made public. More than 400,000 U.S. children had been immunized with Salk's IPV and, as the results of effective protection against this dreaded disease were declared, Americans breathed a collective sigh of relief. The Salk vaccine was declared 90% effective against Types II and III poliovirus and 60 to 70% effective against Type I. Within 2 hours, Salk's IPV was licensed for use. Thanks to guarantees from the National Foundation for Infantile Paralysis (now the March of Dimes), industrial production facilities were already built and ready to operate. The goal was to have five million U.S. children vaccinated by July 1955. Across the Atlantic, some European countries imported the Salk vaccine from the United States whereas others, including Denmark, Sweden, and the Netherlands, began vaccine production in their own government facilities.
Many virologists were of the opinion that Salk's vaccine could not provide long-lasting protection and that this could only be achieved with Sabin's live-attenuated version. Only a live vaccine, it was argued, had sufficient immunogenicity to provide protection. In contrast, an inactivated vaccine would have to be re-administered regularly. Undeterred by Salk's popular success, Cox and Koprowski at Lederle (Koprowski later took his candidate vaccine to the Wistar Institute) and Sabin at the University of Cincinnati continued to work on their live-attenuated virus preparations. Trials of their vaccines took place largely outside the United States because widespread immunization with the Salk vaccine meant that most U.S. children had antibody levels that were too high to enable evaluation of a second vaccine. Instead, Koprowski tested his vaccine in Northern Ireland and in (and around) the Congo, Cox in Latin America, and Sabin in the Soviet Union. By July 1960, more than 15 million Soviet citizens were said to have received Sabin's oral vaccine.
On the basis of these trials, Sabin's vaccine was deemed the better of the two. It was found to confer longer-lasting immunity, so that repeated boosters were not necessary, and acted quickly, immunity being achieved in a matter of days. Taken orally (on a sugar cube or in a drink), the vaccine could be administered more readily than the Salk vaccine, which had to be injected. Most importantly, the Sabin vaccine offered the prospect of passive vaccination because it caused an active infection of the bowel that resulted in the excretion of live-attenuated virus. Thus, through fecal matter and sewage the Sabin vaccine could help to protect those who had not been vaccinated. In August 1960, the U.S. Surgeon General recommended licensing of the Sabin vaccine. The oral vaccine gradually supplanted its rival and by 1968, Salk's vaccine was no longer being administered in the United States, and U.S. pharmaceutical companies had stopped producing it. This interplay—between emerging consensus on the part of health authorities and physicians, and growing commitment on the part of the manufacturing industry to carry out the consensus—is a good example of technological “lock in,” a theory propounded by evolutionary economists.
Despite the switch from the Salk to the Sabin vaccine by the United States, other countries including the Netherlands and Scandinavia continued exclusive use of the Salk vaccine even though the advantages of the live-attenuated vaccine seemed clear-cut. As early as 1962, there were growing suspicions that in a very small number of cases, largely adults, the live-attenuated vaccine could lead to paralytic poliomyelitis. In 1964, an advisory committee established by the U.S. Surgeon General reviewed the incidence of the disease between 1955 and 1961 (when only the Salk vaccine was used) and between 1961 and 1964 (when the Sabin vaccine predominated). They concluded that of the 87 cases of paralytic polio reported in the United States since 1961, 57 were judged “compatible” with having been caused by the attenuated poliovirus regaining its virulence. By the mid-1960s, health officials had to weigh the many benefits of the live-attenuated vaccine against the small but definite risks that were now known to be associated with its use.