Traces of a form of polio derived from a vaccine strain have been found in sewage samples taken from a London sewage treatment plant.
This was announced on Wednesday by the World Health Organization (WHO) and the British authorities.
"It is important to note that the virus was isolated only in environmental samples - no associated case of paralysis was detected", notes the UN organization.
WHO considers it "important that all countries, especially those with a high volume of travel and contact with polio-affected countries and areas, strengthen surveillance in order to rapidly detect any new importation of virus and to facilitate a quick response”.
"Any form of poliovirus, wherever found, poses a threat to children everywhere."
Polio can cause permanent paralysis
Poliomyelitis is a highly contagious disease that invades the nervous system and can cause permanent paralysis.
Wild poliovirus is the best known form of poliovirus.
There is another form of poliovirus that can spread within communities: circulating vaccine-derived poliovirus, or cVDPV.
Although cVDPVs are rare, they have become more common in recent years due to low vaccination rates in some communities.
Read alsoMalek Boutih affected by polio: "My life changed because of a glass of water"
Circulating vaccine-derived polioviruses type 2 (cVDPV2) are the most prevalent, according to the Global Polio Eradication Initiative, a public-private partnership led by national governments with six major partners including the WHO.
959 cases were confirmed worldwide in 2020.
Localized spread
The British Health Security Agency said on Wednesday that the “isolates” had been found “in multiple samples of sewage taken from a London sewage treatment plant between February and June.
This station covers a wide area in the north and east of the British capital, covering a population of nearly 4 million.
Read alsoDiphtheria, tetanus, whooping cough, polio... how Covid-19 has delayed childhood vaccination around the world
“These results suggest that there may be localized spread of poliovirus, most likely among people who are not up to date with their polio vaccinations,” says polio specialist Kathleen O'Reilly.
In recent years, an average of 1-3 poliovirus isolates per year have been detected in sewage samples in the UK.
But these isolates were unrelated.
In the present case, indicates the British health security agency, “the isolates (…) are genetically linked”, making it necessary to study the transmission of this virus in north-east London.
A vaccine that is not used in the UK
According to UK authorities, the most likely scenario is that a recently vaccinated individual entered the UK before February from a country where oral polio vaccine (OPV) has been used in vaccination campaigns.
While the UK stopped using OPV in 2004, several countries, including Pakistan, Afghanistan and Nigeria, have continued to use OPV containing type 2 virus to control outbreaks.
Read alsoPolio eradicated in Africa after 30 years of fight, announced the WHO in 2020
OPV is made from an attenuated form of the live poliovirus which “gives us immunity by growing in the intestine for a short period of time during which it can be detected in the stool”, explains Nicholas Grassly, professor at the Imperial College London.
“This virus can occasionally be transmitted and very rarely it (…) can cause an outbreak of vaccine-derived poliovirus,” he says, noting that OPV was replaced in the UK by an injectable inactivated vaccine in 2004. .