The UK is launching a polio vaccine booster campaign for children in London aged under 10 after confirming that the virus is spreading in the capital for the first time since the 1980s.
The UK Health Security Agency UKHSA has identified 116 polio viruses from 19 sewage samples this year in London. The alert was raised in June.
The levels of the polio virus found since and the genetic diversity indicate that transmission was taking place in a number of London boroughs, the agency said.
In a bid to get ahead of a potential outbreak GPs will invite children aged one to nine for booster vaccine, along with a wider campaign already announced.
Immunisation rates across London are well below the 95 per cent coverage rate that the World Health Organization suggests is needed to keep polio under control.
Thousands of children are killed each year by the spread of polio, which is mainly caused by contamination by faecal matter.
There is no cure, but vaccination brought the world close to ending the wild, or naturally occurring, form of the disease. It affects less than 1 per cent of children who are infected.
The vaccine-like virus that is found in London sewage is mainly found in children who have been vaccinated with a particular type of live vaccine, now only used overseas, which sheds the virus in their faeces.
The harmless virus can transmit between unvaccinated children, and can mutate back into a more dangerous version of the virus.
In an unvaccinated individual, the United States found a case of polio outside New York. The case was genetically linked to a virus seen in London, according to the UKHSA.
The UK is expanding surveillance for polio to other areas outside London.
The risk to the wider population is low because most people are not vaccinated even if rates are below the optimal levels to prevent spread.