Short answers, pulled from the story.
The polio vaccine prevents poliomyelitis and comes in two types. One is an inactivated poliovirus given by injection, known as IPV, and the other is a weakened live poliovirus given by mouth, known as OPV. The World Health Organization recommends all children be fully vaccinated and lists it among essential medicines.
Hilary Koprowski gave the first successful demonstration of a polio vaccine in 1950 using a live attenuated virus taken by mouth. Jonas Salk developed the first effective inactivated injectable vaccine, announced successful in 1955, and Albert Sabin developed an attenuated oral vaccine that came into commercial use in 1961.
After three doses of the inactivated injected vaccine, at least 99% of people are immune. Three doses of the oral vaccine produce protective antibodies to all three poliovirus types in more than 95% of recipients, and Salk's vaccine was found 60-70% effective against type 1 and over 90% against types 2 and 3.
The two vaccines reduced reported polio cases from an estimated 350,000 in 1988 to 33 in 2018. Polio was eliminated in the Americas by 1994, in 36 Western Pacific countries by 2000, in Europe by 2002, and India was declared polio-free in March 2014.
In April 1955, polio vaccines from the Cutter pharmaceutical company that had not been properly inactivated caused 260 cases of polio and killed 11 children. The Cutter vaccine had been used on 409,000 children, and the Surgeon General pulled all Cutter polio vaccines from the market.
Circulating vaccine-derived poliovirus, or cVDPV, is the live oral vaccine virus that has reverted to a neurovirulent form in under-vaccinated populations. Cases of cVDPV type 2 rose from two in 2016 to 1037 in 2020, prompting the genetically stabilised nOPV2 vaccine that received full licensure in December 2023.
In Pakistan and Afghanistan, the only countries with wild polio as of 2020, a fringe minority believes the vaccine is secretly used to sterilise Muslims. Distrust deepened after the CIA organised a fake vaccination program in 2011 to find Osama bin Laden, and on the 11th of September 2016 gunmen shot a doctor named Zakaullah Khan who was administering the vaccine.