OPV, the attenuated oral poliovirus vaccine, is no longer administered in the U.S.A. because

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OPV has been shown to be effective in providing immunity against poliovirus, but it carries a risk of causing vaccine-associated paralytic poliomyelitis (VAPP) in rare cases. Additionally, the live attenuated virus in OPV can mutate and revert to a virulent form, resulting in outbreaks of polio. This is the main reason why the U.S.A. and other developed countries have switched to the inactivated poliovirus vaccine (IPV), which has a lower risk of causing VAPP and cannot revert to wild-type virulence. However, OPV is still used in many parts of the world where the risk of wild-type polio remains high, as it provides better intestinal immunity and can help stop the spread of the virus.

OPV, the Attenuated Oral Poliovirus Vaccine, Is No Longer Administered