Traces of a form of polio derived from a vaccine strain have been found in sewage samples taken from a London wastewater treatment plant, the World Health Organization and British authorities announced on Wednesday.
“Importantly, the virus was only isolated from environmental samples – no associated cases of paralysis were identified,” the WHO said in a statement.
The WHO considers it “important that all countries, especially those with high levels of travel and contact with polio-affected countries and territories, strengthen surveillance to rapidly detect the importation of any new virus and facilitate a rapid response.”
According to the WHO, “any form of poliovirus, wherever it is found, poses a threat to children everywhere.”
Poliomyelitis is a highly contagious disease that affects 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 the community: circulating vaccine-derived poliovirus, or cVDPV. While cVDPVs are rare, they have become more common in recent years due to low vaccination rates in some communities.
Circulating vaccine-derived poliovirus type 2 (cVDPV2) is the most prevalent, according to the Global Polio Eradication Initiative, a public-private partnership led by national governments with six major partners, including WHO.
In 2020, 959 cases were confirmed worldwide.
– “genetically linked” –
The UK Health Safety Agency said on Wednesday that “isolates” were found “in several sewage samples taken from a London sewage treatment plant between February and June.
Poliomyelitis (AFP/Archive – Gal ROMA)
This station covers a vast area in the north and east of the British capital with a population of almost 4 million people.
“These results suggest that local spread of poliovirus is possible, most likely among people who are not aware of 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 not related. In this case, the British Health Security Agency points out, “the isolates (…) are genetically related”, making it necessary to study the transmission of this virus in north-east London.
According to UK authorities, the most likely scenario is that a recently vaccinated person would enter 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, continued to use OPV containing type 2 virus to control outbreaks.
OPV is made from an attenuated form of live poliovirus, which “gives us immunity by growing in the gut for a short period of time during which it can be found in stool,” explains Nicholas Grassley, professor at Imperial College London.
“This virus can sometimes be transmitted and very rarely (…) can cause a vaccine-derived poliovirus outbreak,” he says, noting that OPV was replaced in the UK by an injectable inactivated vaccine in 2004.