The UK Heath Security Agency (UKHSA) is reporting several traces of poliovirus have been detected in London sewage samples over the past few months. So what does this mean? And should anyone be worried?
What has been found?
For some years sewage samples in London have been routinely gathered and tested for a wide range of pathogens. In this instance the UKHSA reports several samples of "vaccine-derived" poliovirus type 2 (VDPV2) were detected between February and May at a single sewage treatment center in London.
VDPV2 is a type of poliovirus that comes from an oral polio vaccine, which utilizes a live form of the virus, unlike the injectable polio vaccine that uses an inactivated form of virus.
For a short period of time after being administered the oral vaccine a person can potentially transmit live infectious virus through their feces. VDPV2 is defined as a strain of poliovirus that has been transmitted from human to human after originating in the oral vaccine.
Why are we still using live poliovirus in some vaccines?
The United Kingdom switched to the injectable inactivated polio vaccine in 2004. As Imperial College London vaccine researcher Nicholas Grassley explains, the oral polio vaccine is still used in poorer countries as it is easy to administer and offers more rapid protection in the face of acute disease outbreaks.
“Oral polio vaccine is easy to administer and gives good gut immunity that protects against infection and poliovirus shedding in stool if exposed, “ said Grassley. “It is needed to stop polio outbreaks in countries with poor sanitation where food and water may be contaminated with human waste.”
Why is this particular sewage finding important?
The UKHSA notes it is not uncommon for "vaccine-like" strains of poliovirus to be detected in London sewage samples. Usually between one and three times a year there are traces of this kind of poliovirus detected, and these are thought to be from individuals vaccinated in another country who recently returned to London.
What is unusual about these detections is there have been several positive detections coming from the same sewage facility over the course of a few months. Genomic testing has subsequently revealed these positive samples are all related, suggesting the virus has been spreading through one or more individuals in London over recent months. This kind of poliovirus community transmission in London has not been detected since the 1980s.
How has this happened without anyone noticing?
The vast majority of poliovirus infections are asymptomatic. Around 70 percent of people who contract the virus will never know they have it and less than one percent of infections move from the gut to the nervous system to cause the more common motor paralysis seen in past polio outbreaks.
So far there have been no reports in London, or the greater United Kingdom, of poliomyelitis. This means at this stage the virus has likely only moved through a small volume of people without generating any severe disease.
But aren’t most people in London already vaccinated against polio?
Current UK vaccination recommendations suggest all children under five complete a three-dose schedule of the inactivated injectable polio vaccine. This is covered as part of a six-in-one vaccine called DTaP/IPV/Hib/HepB, administered to most children at eight, 12 and 16 weeks of age, with a booster commonly given at three years.
It's estimated around 86.6 percent of all people in London are immunized against polio. Of children aged five and under, coverage in London is 91.2 percent, slightly below the UK average of 95 percent.
This means nearly 10 percent of all children aged under five in London are potentially susceptible to the severe effects of polio. Kathleen O’Reilly, an infectious disease expert, hypothesizes this localized polio spread that has been detected in wastewater is moving through an unvaccinated community somewhere in north London.
“These findings suggest that there may be localized spread of poliovirus, most likely within individuals that are not up to date with polio immunizations,” O’Reilly said. “The most effective way to prevent further spread is to check vaccination histories, especially of young children, to check that polio vaccination is included.”
So should anyone be worried?
Not at this stage. Although this detection of potential community transmission is relatively unprecedented it still is occurring amongst a city of highly vaccinated children and there are currently no signs of severe disease.
If anything, this polio detection should serve as a potent reminder that there are still a large number of infectious diseases out there in the world and vigilance is needed to maintain herd immunity through widespread vaccination. David Heymann, from the London School of Hygiene & Tropical Medicine, also points out this is a reminder that certain diseases such as polio may have dropped off the radar for parents in Western countries but they are still very much present in many parts of the world. And in a globalized world if there is polio somewhere it can very quickly move to the other side of the planet.
“Its presence in the reminds us that polio eradication has not yet been completed in the world,” added Heymann. “The high vaccination coverage using inactivated polio vaccine in the UK will limit the spread of vaccine derived polio and protect those who have been vaccinated against polio paralysis.”
