The UK's health authorities on Wednesday called on parents to have their children updated on polio vaccines as they investigate a possible outbreak of the disease.
The UK Heath Security Agency (UKHSA) said several closely-related polioviruses were found in sewage samples taken between February and May from the London Beckton Sewage Treatment Works, which serves around 4 million people in north and east London.
It also said the virus has evolved into a "vaccine-derived" poliovirus type 2 (VDPV2) that has the potential to spread in communities and in rare occasions can "cause serious illness, such as paralysis, in people who are not fully vaccinated."
Dr. Vanessa Saliba, consultant epidemiologist at the UKHSA, said the overall risk to the public is "extremely low."
With the last case of wild polio confirmed in 1984, the UK has been declared polio-free since 2003 by the World Health Organisation (WHO).
According to the UKHSA, it's normal to find one to three "vaccine-like" polioviruses in routine checks of sewage samples each year. This is because people who travel to the UK after having the live oral polio vaccine overseas, which contains weakened polioviruses, briefly shed traces of the vaccine-like poliovirus in their faeces. However, these have always been one-off cases.
The UKHSA is now working on the theory that there has been some spread between closely-linked individuals in north and east London who are now shedding the type 2 poliovirus strain in their faeces.
Experts are looking at the possibility that just one family may be affected, though it is unclear how many people need to be infected for polio to be detected in sewage samples.
According to the WHO, an excreted vaccine virus may stay alive for an extended period of time where the population is "seriously under-immunized" and mutate into "a form that can paralyse."
The UKHSA stressed that the virus has only been detected in sewage samples and no cases of paralysis have been reported.
It is now investigating the extent of community transmission and has established a “national incident” to check for cases elsewhere as a precaution.
The polio vaccine is given on the NHS when a child is 8, 12, and 16 weeks old as part of the 6-in-1 vaccine. It is given again at 3 years and 4 months old as part of the 4-in-1 (DTaP/IPV) pre-school booster, and at 14 as part of the 3-in-1 (Td/IPV) teenage booster.
All of these vaccines need to have been given for a person to be considered fully vaccinated, though babies who have had two or three doses will have substantial protection.
The latest figures show that by the age of 2 in the UK, almost 95 percent of children are up-to-date with the doses. However, this drops to just under 90 percent in London.
When it comes to the pre-school booster, just 71 percent of children in London have had it by the age of 5.
Jane Clegg, chief nurse for the NHS in London said the NHS will begin contacting parents of children aged under 5 in London "who are not up to date with their polio vaccinations to invite them to get protected."