Environmental surveillance for polio
Published 8 April 2025
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The UK Health Security Agency (UKHSA), working with the World Health Organization (WHO) Polio Global Specialised Laboratory at the Medicines and Healthcare products Regulatory Agency (MHRA), conducts routine environmental surveillance for polio in England as part of our commitment to the Global Polio Eradication Initiative (GPEI).
Environmental surveillance
Environmental surveillance consists of the collection of raw sewage samples from a number of locations which are then tested for the presence of enteroviruses including poliovirus. The purpose of the sewage surveillance is to provide an early warning system of poliovirus importations which may then lead to community transmission. In this latter scenario the poliovirus detected would be expected to persist for a period of weeks and months.
UKHSA has followed the WHO guidance on environmental surveillance for poliovirus and established protocols to test samples using the latest advice. Currently this includes obtaining the nucleotide sequence of the gene coding for the virus (capsid VP1 protein) which is essential for virus classification. The UKHSA National Polio Guidelines outline response actions to be undertaken depending on the virus detected, which could be vaccine-like poliovirus showing none or few nucleotide changes from the live-attenuated oral polio vaccine (OPV) strain, indicative of no or very short-lived replication/transmission in vaccinees and/or close contacts, or vaccine-derived poliovirus (VDPV) which show higher sequence drift from the OPV vaccine strain, suggestive of longer-term replication/transmission in one or more individuals. The third type is wild poliovirus (WPV) which might be imported from areas of Pakistan and Afghanistan where WPV type 1 (WPV1) is still circulating or has been inadvertently released from laboratory facilities using WPV for vaccine production and/or quality control and research activities.
As part of this routine polio surveillance, we do expect to detect polioviruses from time to time. These are normally one-off findings due to:
1. Individuals being vaccinated overseas with OPV and entering the UK where they would briefly ‘shed’ traces of the ‘vaccine-like’ poliovirus in their faeces. Several countries offer OPV on exit, for example, Pakistan, as part of their response to polio outbreaks
2. Individuals who are immunosuppressed may shed VDPV for a long period of time, named immunodeficiency-related vaccine-derived poliovirus (iVDPV) in these cases
3. A visitor from a country where VDPV has been circulating
These one-off or brief detections do not require any public health action.
In England, sewage samples are collected monthly from 28 sewage treatment sites (see map) in collaboration with the Water Companies and are sent to the MHRA laboratory for processing, with additional testing capacity provided by NHS Lothian.
Figure 1. Map of sewage treatment sites
The sewage sampling sites were selected on the basis of the following criteria:
1. Areas with low vaccine coverage for the childhood immunisation programme
2. Areas with pockets of under vaccinated communities
3. Areas with higher risk of importation from a country where WPV1 or VDPV is circulating or where OPV is still used
The UKHSA is working closely with partners in the Devolved Administrations who are at different stages of establishing routine environmental surveillance for polio.
Main findings
As part of routine surveillance, it is normal for a handful of ‘vaccine-like’ polioviruses to be detected each year in UK sewage samples but these are usually single or very brief detections that do not trigger any public health action. Here we summarise key findings from detections that required a national incident to be established in order to coordinate the investigation and response.
2022
In June 2022, UKHSA announced that environmental surveillance for polio had been persistently detecting poliovirus in sewage samples collected from the London Beckton Sewage Treatment Works (STW) since February 2022. A UKHSA national enhanced incident response was established, and environmental surveillance was expanded to cover a range of sites around England. In August 2022 it was formally confirmed that the UK had a ‘circulating’ vaccine-derived polioviruses type 2 (cVDPV2) based on the detection of the same isolate for more than 60 days. Evidence also suggested that the virus detected in London was genetically linked to the poliovirus detected in Israel and the US. In response, the Joint Committee on Vaccination and Immunisation (JCVI) recommended that polio vaccine booster doses should be offered to children aged 1 to 9 years across London. The last detection linked to this incident was in sewage collected in London in November 2022. The WHO removed the UK from the list of polio ‘infected’ countries in December 2023 after there had been no detections for a period of 12 months. The full details of the findings were published in The Lancet.
2024
Testing of sewage samples collected in November 2024 identified cVDPV2 in one sample from East Worthing STW, which covers some of the West Sussex local authority and adjacent areas, 2 samples from Leeds Knostrop STW, one sample from London Beckton STW and one sample from London Crossness STW. At that time, sampling at the sites where the vaccine-derived poliovirus type 2 (VDPV2) was detected was increased to fortnightly to inform a timely investigation and any necessary response.
Testing of sewage samples in December detected VDPV2 from a single sample collected from London Beckton STW on 10 December. All other sites tested negative for poliovirus in December.
So far in 2025, linked VDPV2 was detected from a single sample collected from Shoreham STW on 20 January, with subsequent samples from this site testing negative. All other sites including all those with VDPV2 detections in November have tested negative for poliovirus to date in 2025.
The poliovirus isolates found in the UK are genetically linked to a VDPV2 that has been widely circulating in several African countries in recent months, unrelated to any previous poliovirus found in the UK. All strains detected from the 5 countries in Europe are genetically linked between them, classifying these isolates as cVDPV2 as per GPEI requirements for VDPV classification and reporting.
To date, there is no evidence of community transmission in the UK and the risk to the public is low. Whilst there is currently no evidence of community transmission, VDPVs do have the potential to spread, particularly in communities where vaccine uptake is low. On rare occasions it can cause paralysis in people who are not fully vaccinated. These latest findings are a timely reminder of the importance of vaccination to protect against poliovirus and ensuring any children not fully up to date with their polio vaccinations are caught up. The UKHSA is coordinating the public health investigation and response to these detections with system partners at the national, regional and local level.
A rapid communication has been published in Eurosurveillance which provides an overview of the recent linked detections of VDPV2 in 5 countries in Europe: Spain, Poland, Germany, the UK and Finland.
Figure 2. Flowchart of sewage surveillance and its role in the detection and prevention of polio
