Polio is transmitting between people in parts of London and mutated strains of the virus that can cause paralysis have been detected in sewage, a leading immunisation expert warned on Thursday.
Professor Sir Andrew Pollard urged parents in the capital to bring their children forward to get a polio booster jab.
All children aged one to nine living in Greater London will be offered a polio vaccine after the virus was detected in waste water.
It has been found 116 times in the boroughs of Barnet, Brent, Camden, Enfield, Hackney, Haringey, Islington, and Waltham Forest since February.
“It is absolutely the case that when we find the occasional polio virus in sewage, which is usually introduced from another country, that then disappears,” Sir Andrew told BBC Radio 4.
“What has happened here is that since February there’s been consistent isolation of viruses over that period.
“I think most importantly, some of those viruses have mutated towards the types of strains which we know can cause paralysis.
“And so the only way that you can have persistent appearance in sewage is if it is transmitting between people in parts of London.”
The UK Health Security Agency said most of the samples detected in sewage so far are the safe vaccine form of polio, but “a few” have mutated enough to be dangerous.
An urgent immunisation campaign was announced on Wednesday and will see nearly a million children offered a booster vaccine - including those already up to date with their jabs.
Parents and carers will be contacted by their GP about the jab within the next month.