Everyone in the US who didn’t explicitly refuse the vaccine is immunized against polio. A vaccine has been available since the 50s, it has been recommended by the CDC since the early 60s. They used to give Oral Polio Vaccine (OPV) which was a live attenuated (i.e. weakened) vaccine but there were a very small number of cases where people contracted the disease from the weakened virus. Once Inactivated Polio Vaccines (IPV) became available the CDC recommended those instead and OPV ceased to be offered in the US and most developed countries. OPV is still available in other parts of the world where polio continues to be an issue. Most polio vaccination in the US takes place as part of combination vaccines which also protect against diphtheria, tetanus, pertussis, and either helatits B or Hib (depending on the manufacturer/brand).
(Also, before any anti-vaxxer says anything about why we are still immunizing for polio if it is no longer an issue here, it is because anytime countries/areas have gotten complacent in their immunization efforts, polio cases have reappeared and in certain cases in African nations, there have been full#on regional outbreaks).