Although oral polio (3 viruses) can stop Polio, the problem is that in one of a million cases, the weakened virus mutates back to the real thing.
Not a big problem in countries where most people have gotten the vaccine (in the USA, about 7 cases a year) but a big problem in countries where Mullahs read the UKGuardian and similar anti vaccine screeds and told their people that vaccines were a western plot to destroy them and their kids. Result? Lots of unvaccinated kids to play with the kids who do get the vaccine, and secondary epidemics
So the WHO and others are changing the way vaccines are given, including giving kids one shot of the old fashioned shot (killed virus) vaccine.
for my later reading.