LONDON — Every child in London age 1 to 9 should be offered a booster dose of polio vaccine, the British health authorities said on Wednesday, after traces of the virus turned up in sewage samples across a quarter of the city’s boroughs, though no individual cases of the disease have been reported.
Health authorities declared a national incident in June — a designation that is used to underscore the potential seriousness of the issue — after sewage samples suggested the virus was spreading in London. Now, officials said, samples indicated that it had spread beyond a close network of a few individuals.
Normally, routine surveillance of sewage in Britain picks up the virus once or twice a year, the U.K. Health Security Agency said in a statement, but between February and July, 116 samples of type 2 poliovirus were detected in samples from eight London boroughs in the north and east of the city.
“No cases of polio have been reported and for the majority of the population, who are fully vaccinated, the risk is low,” Dr. Vanessa Saliba, a consultant epidemiologist at the health agency, said in the statement.
“But we know the areas in London where the poliovirus is being transmitted have some of the lowest vaccination rates. This is why the virus is spreading in these communities and puts those residents not fully vaccinated at greater risk.”
Polio, which can cause paralysis, was once a profoundly feared childhood disease but has practically been eradicated by vaccination. Britain’s last known case was in 1984, and a case discovered in Rockland County, N.Y., last month was the first in the United States in nearly a decade. Wild poliovirus has been eliminated from every country in the world except Afghanistan and Pakistan.
The traces found in London’s sewage, like the case in New York State, involved poliovirus derived from the oral polio vaccine, which uses a weakened live form of the virus that immunized people excrete for a short time.
In rare cases, in communities with low vaccination coverage, the virus can spread. In very rare cases — generally where vaccination protection is low enough that it circulates for at least a year, according to the World Health Organization — it can gradually mutate back into a form that can paralyze.
Most of the London sewage samples contained a form of virus still very close to the harmless version in the vaccine, the U.K. Health Security Agency said, but a few carried enough mutations to cause severe disease.
The oral polio vaccination is no longer routinely given in Britain, which, like the United States, now favors a vaccine that uses inactivated — in effect, dead — virus.
The U.K. Health Security Agency said childhood vaccination uptake in London was lower than the rest of the country. Recent figures in London suggest a broader immunization coverage of 86.6 percent, according to the Global Polio Eradication Initiative, which should generally be sufficient to prevent widespread transmission of the virus.
“While the majority of Londoners are protected from polio, the N.H.S. will shortly be contacting parents of eligible children aged one to nine years old to offer them a top-up dose to ensure they have maximum protection from the virus,” said Jane Clegg, the chief nurse for Britain’s National Health Service in London, adding that medics were already reaching out to parents and carers of children whose vaccinations were not up to date.
The U.K. Health Security Agency said on Wednesday that it was working closely with the World Health Organization, as well as health agencies in New York and Israel, to investigate the links between the poliovirus outbreak in London and recent episodes in Israel and the United States.
Many countries globally, including the United States and Israel, already provide an additional polio vaccine booster as part of their childhood vaccination schedule. Britain routinely offers a preschool booster vaccine, which includes polio, to children at the age of 3 years and 4 months old.
The British health agency also said on Wednesday that it had already increased sewage surveillance across London, and that 10 to 15 sites across the country would be examined to ascertain whether poliovirus had spread outside of London.
Apoorva Mandavilli contributed reporting from New York.