Traces of a form of polio derived from a vaccine strain have been found in samples of wastewater taken from a London sewage treatment plant, the World Health Organization and British authorities have announced. “It is important to note that the virus was isolated only from environmental samples – no associated cases of paralysis were detected,” the WHO said.
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”. According to the WHO, “any form of poliovirus, wherever found, poses a threat to children everywhere”.
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. “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 no longer 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.
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 has been replaced in the UK by an injectable inactivated vaccine. in 2004.