We have a similar issue with poliovirus in sewage here. A few samples crop up most years, but there's been something of a spike recently. The virus found here has been determined to be vaccine-derived from individuals who have been vaccinated with a live form of the virus. We haven't used live virus vaccine here for almost twenty years, so it seems that the virus found has come from someone who had the live virus oral vaccine overseas then came here. That wouldn't be a problem in theory, but a vaccine-derived live virus is theoretically capable of evolving into a pathogen. There have been no recorded cases of polio here in recent years. It seems that the risk is minimal (unless there are undetected cases in the community: none found so far), so the advice has been to make sure that vaccinations are up to date, especially in children. I have a very hazy recollection of having had only three out of the course of four vaccinations, as was in those days, well before I was ten. I won't be worrying too much but I'll check on my grandson with his mum and dad when they arrive here tomorrow.