The polio virus is making a comeback

in London

.

In fact,

evidence of the spread of the virus

in different regions of the city has been found, although no case of infection has been confirmed. 

So the British health authorities have announced that they will offer a

booster dose for polio

to children aged 1 to 9 in the English capital. 

In a statement on Wednesday, the British Health Security Agency said it had detected polio viruses derived from the oral polio vaccine

in the sewage waters of eight

London boroughs, but had not identified any cases.

Analysis of the virus samples suggested that "the transmission went beyond a tight network of a few individuals," but found

no one infected

. 

According to

the World Health Organization

, only out of 200 polio infections lead to paralysis, while most people show no symptoms. 

"This will ensure a high level of protection from paralysis and help reduce further spread," the agency said.

Most people in the UK are vaccinated against polio in childhood, and the agency said the risk to the general population is low.

The agency is currently working closely with

US and Israeli health authorities

and WHO to investigate links between polio viruses detected in these two countries. 

"We know that the areas of London where poliovirus is transmitted have

some of the lowest vaccination rates

," said Dr Vanessa Saliba, UK Health Safety Agency epidemiologist consultant. 

"For this reason the virus is spreading in these communities and puts residents who are not fully vaccinated at greater risk," he added. 

Polio is a disease that

often spreads in water

and mainly affects children under 5 years of age.

It has mostly been eliminated by developed countries, but outbreaks remain in Pakistan, Afghanistan and parts of Africa. 

Initial symptoms

include fever, fatigue, headache, vomiting, and muscle stiffness.

Among people paralyzed by the disease, death can occur in up to 10% of cases when the respiratory muscles become paralyzed. 

In rare cases, the live virus contained in the oral polio vaccine used in the global effort to eradicate the disease can mutate into new forms potent enough to trigger new outbreaks. 

The vaccination booster in London will use injected polio vaccines that do not carry this risk.