Summary: Whilst initial immunity is developing after vaccination, attenuated polio viruses can be excreted and cause a rare circulating vaccine-derived poliovirus (cVDPV) outbreak. 21 outbreaks have occurred in the last 20 years, with 760 cases, out of 3 billion vaccinated and 10 million polio cases averted. cVDPV is averted by getting as many people in the community vaccinated within a 12-month window as possible. It naturally stops being a risk as the vaccinated population approaches 100%.
https://www.who.int/westernpacific/news/q-a-detail/what-is-v...
Summary: Whilst initial immunity is developing after vaccination, attenuated polio viruses can be excreted and cause a rare circulating vaccine-derived poliovirus (cVDPV) outbreak. 21 outbreaks have occurred in the last 20 years, with 760 cases, out of 3 billion vaccinated and 10 million polio cases averted. cVDPV is averted by getting as many people in the community vaccinated within a 12-month window as possible. It naturally stops being a risk as the vaccinated population approaches 100%.