Almost a million children aged one to nine across London are to be offered a polio vaccine to try to prevent the spread of the virus.
Health officials warned there has been “some transmission” of the virus in the capital after detecting poliovirus in sewage samples.
Polio, which was officially eradicated in the UK in 2003, can cause paralysis in rare cases and can be life-threatening.
While there have been no confirmed cases, officials sounded the alarm over the rising number of samples found in sewage in London.
The UK Health Security Agency (UKHSA), working with the Medicines and Healthcare products Regulatory Agency (MHRA), found poliovirus in sewage samples in London boroughs including Barnet, Brent, Camden, Enfield, Hackney, Haringey, Islington and Waltham Forest.
It was first detected at Beckton sewage treatment works earlier this year.
Officials said levels of the virus found in sewage and their “genetic diversity” suggests “some virus transmission in these boroughs”.
As a result officials are to launch a rapid vaccination programme among youngsters in London, where there are lower levels of uptake of the vaccine.
Vaccination rates in the capital are well below recommended levels and there is a risk that “under-vaccinated” children can pass the infection to others.
Youngsters can be responsible for “silent transmission” of polio, which means that they may have the virus but not show any symptoms.
Children in London aged one to nine who are not yet fully vaccinated will be offered a catch-up dose, while those who have already been fully vaccinated will be offered a booster.
Youngsters will be offered a jab within the next four weeks with officials hoping to vaccinate all those invited within six weeks.
Officials hope a rapid vaccination campaign will boost antibodies to a high degree which should interrupt transmission of the virus.
It is also hoped that the campaign will help prevent any potential cases of paralysis.
While it is normal for the virus to be picked up as isolated cases and not detected again, experts have raised the alarm after several genetically linked viruses were found in sewage samples since February.
Previously, the virus had been picked up when a person vaccinated overseas with the live oral polio vaccine (OPV) returned or travelled to the UK and briefly shed traces of the vaccine-like poliovirus in their faeces.
However, the virus in the recent samples had evolved in England and is now classified as a “vaccine-derived” poliovirus type 2 (VDPV2).
VDPV is a strain of the weakened poliovirus, that was initially included in the oral polio vaccine, which has changed over time and behaves more like the “wild” or naturally occurring virus.
This means it can be spread more easily to people who are unvaccinated and who come into contact with the faeces or coughs and sneezes of an infected person.
In total 116 samples have been found since February, the UKHSA said, although officials stressed this does not equate to 116 cases as they may have found samples from the same person on multiple occasions.
A recommendation from the Joint Committee on Vaccination and Immunisation for the targeted vaccine campaign in the capital was accepted by Health Secretary Steve Barclay.
The NHS in London will contact parents when it their child’s turn to get the vaccine, with parents urged to take up the offer “as soon as possible”.
The programme will start in the areas where the virus has been detected in sewage and then be extended across all London boroughs.
Other countries, including the USA and Belgium, already offer an additional dose of the polio vaccine.
The virus has not yet been found outside London but officials are stepping up surveillance across the rest of the country.
The UKHSA said that nationally the overall risk of paralytic polio is considered “low”.
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