Salk

The two polio vaccines differ in ways that are heavily examined. Salk IPV is killed, given intramuscularly, induces serum IgG that prevents viraemia and therefore paralytic poliomyelitis, but generates little intestinal mucosal IgA, so vaccinated people can still be transiently infected and shed virus in stool. It carries no risk of vaccine-associated paralytic poliomyelitis and is safe in immunodeficient patients and their household contacts. The Sabin oral polio vaccine is live attenuated, induces both IgG and mucosal IgA, interrupts faecal-oral transmission, is cheap and needs no needle, and provides contact immunity through shedding — advantages that made it the mainstay of global eradication — but it can revert to neurovirulence, causing vaccine-associated paralytic poliomyelitis at roughly one per two to three million doses, and can generate circulating vaccine-derived polioviruses. As wild poliovirus has receded, that residual risk led the United States to move to IPV alone in 2000. Poliovirus itself is a non-enveloped, positive-sense RNA enterovirus destroying anterior horn motor neurons.