Enlarge / A health worker administers a polio vaccine to a child out of Kabul Afghanistan on May 17, 2016. (credit: Getty | Anadolu Agency )

A vaccine-derived version of poliovirus has repeatedly surfaced in London sewage over the past several months, suggesting there may be a cryptic or hidden spread among some unvaccinated people, UK health officials announced Wednesday .

No polio cases have been reported so far, nor any identified cases of paralysis. But sewage sampling in one London treatment plant has repeatedly detected closely related vaccine-derived polioviruses between February and May. This suggests "it is likely there has been some spread between closely-linked individuals in North and East London and that they are now shedding the type 2 poliovirus strain in their feces," the UK Health Security Agency (UKHSA) said.

Though the current situation raises alarm, the agency notes that it's otherwise common to see a small number of vaccine-like polioviruses pop up in sewage from time to time, usually from people who have recently been vaccinated out of the country. This is because many countries use oral polio vaccines that include weakened (attenuated) polioviruses, which can still replicate in the intestines and thus be present in stool. They can also spread to others via poor hygiene and sanitation (i.e., unwashed hands and food or water contaminated by sewage), which can become concerning amid poor vaccination rates.