The Salk vaccine had been 60–70% effective against PV1 (poliovirus type 1), over 90% effective against PV2 and PV3, and 94% effective against the development of bulbar polio. Soon after Salks vaccine was licensed in 1955, childrens vaccination campaigns were launched. In the U.S, following a mass immunization campaign promoted by the March of Dimes, the annual number of polio cases fell from 35,000 in 1953 to 5,600 by 1957. By 1961 only 161 cases were recorded in the United States.A week before the announcement of the Francis Field Trial results in April 1955, Pierre Lépine at the Pasteur Institute in Paris had also announced an effective polio vaccine. Safety incidents
In April 1955, soon after mass polio vaccination began in the US, the Surgeon General began to receive reports of patients who contracted paralytic polio about a week after being vaccinated with Salk polio vaccine from the Cutter pharmaceutical company, with the paralysis limited to the limb the vaccine was injected into. The Cutter vaccine had been used in vaccinating 200,000 children in the western and midwestern United States. Later investigations showed that the Cutter vaccine had caused 40,000 cases of polio, killing 10. In response the Surgeon General pulled all polio vaccines made by Cutter Laboratories from the market, but not before 250 cases of paralytic illness had occurred. Wyeth polio vaccine was also reported to have paralyzed and killed several children.
The fact that the CIA organized a fake vaccination program in 2011 to help find Osama Bin Laden is an additional cause of distrust. In 2015, the WHO announced a deal with the Taliban to encourage them to distribute the vaccine in areas they control. However, the Pakistani Taliban was not supportive. On 11 September 2016, two unidentified gunmen associated with the Pakistani Taliban, Jamaat-ul-Ahrar, shot Zakaullah Khan, a doctor who was administering polio vaccines in Pakistan. The leader of the Jamaat-ul-Ahrar claimed responsibility for the shooting and stated that the group would continue this type of attack. Such resistance to and scepticism of vaccinations has consequently slowed down the polio eradication process within the two remaining endemic countries. Travel requirements
Travellers who wish to enter or leave certain countries must be vaccinated against polio, usually at most 12 months and at least 4 weeks before crossing the border, and be able to present a vaccination record/certificate at the border checks. : 25–27  Most requirements apply only to travel to or from so-called polio-endemic, polio-affected, polio-exporting, polio-transmission, or high-risk countries. As of August 2020, Afghanistan and Pakistan are the only polio-endemic countries in the world (where wild polio has not yet been eradicated). Several countries have additional precautionary polio vaccination travel requirements, for example to and from key at-risk countries, which as of December 2020 include China, Indonesia, Mozambique, Myanmar, and Papua New Guinea.