Because lots of people continue to grow and shed vaccine strain polioviruses in stool for months to years afterwards, and some strains eventually mutate to become dangerous again.
CDC was very cagey about its plans for sewage surveillance after one young man in New York developed paralytic polio. They limit such surveillance to areas around outbreaks, said a CDC spokesperson. WHY? Because they know they will find lots of polio in the wastewater around the country—and they do not want to alert you to this until they have a polio vaccine campaign or drug to roll out.
Up to 12 months after an oral polio vaccine campaign was conducted in Finland in 1985, 5 of 93 samples detected vaccine-strain polio in sewage. The Finns stopped looking after 12 months:
The efficacy of the vaccination campaign in regard to elimination of the epidemic type 3 strain was evaluated by a follow-up study on viruses in sewage waters continued for 12 months through the subsequent expected season of poliomyelitis. Several types of enteroviruses, including five vaccine-related poliovirus strains, were identified in the 72 virus-positive specimens out of 93 studied. No wild-type polioviruses were found, indicating the success of the campaign.
Most polio cases in the world are due to vaccine strains. The “wild-type” or natural polio only is found in 2 countries presently, Afghanistan and Pakistan, while the vaccine strain viruses are causing polio in 30 countries, including in Europe.