UK health authorities have concluded that there has been human-to-human transmission of the polio virus in certain areas of London in recent months.
The disease was eradicated from the United Kingdom in 2003, so the signs detected have sufficiently alarmed the Health Security Agency (UKHSA) to declare a "national incident".
This is the step prior to an emergency declaration, and requires a detailed investigation of the alert.
Between February and May of this year, several genetically very similar samples of the virus were detected at the Beckton wastewater treatment plant, east of the British capital.
The plant serves more than four million inhabitants.
Until just over a decade ago, the UK provided the oral polio vaccine, which contained a “weakened” version of the virus capable of stimulating the immune system.
This type of vaccine is already only used in a limited number of countries.
In the West, immunization against polio is now done with "inactivated vaccines," which contain a dose of the already killed pathogen.
They require booster injections to effectively stimulate the defenses, but they avoid the risk of the virus mutating and becoming deadly.
It is normal, in routine inspections, that every year some sample of the oral vaccine virus is found in British wastewater, still very present in campaigns in countries such as Pakistan, Afghanistan or Nigeria.
The conclusion, always, is that it comes from someone who has brought the virus with them to the United Kingdom.
The seriousness of the recent discovery lies in the fact that the samples detected, similar and continuous over a period of time, correspond to a Type 2 poliovirus (VDPV2), a "vaccine-derived" mutation that can cause serious disease, such as paralysis, in people who are not vaccinated.
“Vaccine-derived polioviruses are extremely rare, and the health risk to the public remains extremely low,” said Dr Vanessa Saliba, UKHSA Epidemiological Adviser.
"However, they have the ability to spread, especially in those communities where the vaccination rate is low," said Saliba, who has asked parents whose children do not have their immunological card up to date to see a pediatrician immediately.
The vaccine is supplied as a three-dose course.
Before turning one year old;
at three, and, finally, at 14. If 92% of minors throughout the United Kingdom have completed their immunization, in the case of London the figure is 86%.
Although the United Kingdom is considered by the World Health Organization to be a country where polio is eradicated,
"Most Londoners are protected from polio and don't need to take extraordinary measures," said Jane Clegg, the chief nurse at the NHS in London.
"But the NHS is going to start contacting parents of children under five who don't have their cards up to date to encourage them to do so," Clegg said.
So far, the health authorities have not treated any patient with the symptoms of the disease.
Polio is usually spread by people who have not washed their hands after defecation and touch food or water consumed by others.
In rare cases, transmission can occur through coughs or sneezes.
Most of those infected do not develop any symptoms, or simply experience a flu-like illness for two or three weeks.
But in a few rare cases (one in 100 to one in 1,000), the virus can attack nerves in the spine or base of the brain, causing paralysis, usually in the legs.
If it affects the muscles that control the respiratory system, its consequences can be fatal.
The health authorities of the United Kingdom have extended the investigation to areas bordering the wastewater treatment plant where the samples were detected, and have put the health workers on alert to monitor the possible appearance of a patient with symptoms.
