The UK's Joint Committee on Vaccination and Immunisation has advised that polio vaccine booster doses be offered to children aged 1 to 9 across London after more poliovirus was detected in sewage in the city, according to the country's Health Security Agency (UKHSA).
Health authorities have asked all London systems to support delivery as soon as possible, starting no later than August 15, with children under 10 years old having been offered vaccination by the end of September.
"It will ensure a high level of protection from paralysis," said Vanessa Saliba, consultant epidemiologist at UKHSA. "This may also help stop the virus spreading further."
Saliba advised all children to have a dose of polio vaccine now, "whether it's an extra booster dose or just to catch up with their routine vaccinations."
Vaccine-like type-2 poliovirus (PV2) isolates were found in sewage samples collected from the London Beckton sewage treatment works in February and were found in several parts of the city's boroughs, said the county's health authority.
The risk of getting poliovirus remains extremely low, but the chance of getting ill from polio is higher if you are not fully vaccinated, it said.
Should poliovirus be worried?
Polio is a highly infectious viral disease that largely affects children under 5 years of age, according to the World Health Organization.
The virus is transmitted person-to-person and spread mainly through contact with the poo of an infected person or, less frequently, through coughs or sneezes.
Around 8,000 people developed paralysis in the UK every year before a polio vaccination program was introduced in the country, according to BBC Science Focus Magazine. And the last case of polio in the UK was in 1984.
There is no treatment for polio, said the UK's National Health Service (NHS), and the best way to prevent the paralytic disease is to get vaccinated.
The NHS also advises washing hands with soap and water or using hand sanitizer regularly throughout the day to help avoid catching and spreading viruses.
The health authorities declared a national incident after finding evidence suggesting local spread of poliovirus in London in June.