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Polio is an infectious disease caused by the poliovirus.

Owing to a global vaccination and eradication programme, most countries have been declared polio-free, and the disease remains endemic in just Afghanistan and Pakistan.

Polio vaccination

Two types of vaccines have been used in vaccination programmes targeting polio: the oral polio vaccine (OPV) and the inactivated polio vaccine (IPV). Both vaccines provide excellent protection against paralysis – a rare but severe symptom – caused by poliovirus.

The OPV contains a live, attenuated (weakened) form of the poliovirus. This genetically engineered form of the poliovirus will still prompt an immune response but is usually not strong enough to cause illness.

An OPV-vaccinated person will shed viral material in their faeces (poo) for a short period of time after vaccination. If another person (usually unvaccinated) comes into contact with the infected faeces, they may become infected with a vaccine-like poliovirus.

Conversely, the IPV-vaccinated person does not shed viral material in their faeces. This means there is no risk of an IPV-vaccinated person transmitting a vaccine-related poliovirus on to another person.

Vaccine-derived poliovirus

OPVs contain a live (albeit weakened) vaccine-like poliovirus and therefore retain the ability to mutate. Over time, these mutations can accumulate, and may eventually result in a vaccine-derived poliovirus that has regained the ability to cause polio disease.

The accumulation of mutations increases each time the virus is passed (transmitted) from one person to another.

Most countries have stopped using the OPV and have switched to the IPV as it does not create a risk of vaccine-derived poliovirus, including the UK which has used the IPV exclusively since 2004.

Both types of vaccine, the OPV and the IPV, offer protection against vaccine-derived poliovirus.

Detection of vaccine-derived poliovirus in London sewage samples

As part of the UK’s commitment to the World Health Organization (WHO) global polio eradication programme, the WHO Global Specialised Polio Laboratory located at the National Institute for Biological Standards and Control (NIBSC) conducts routine surveillance for wild (naturally occurring) poliovirus and vaccine-like polio viruses. This usually results in a few unrelated poliovirus isolates being identified each year.

Between February and June 2022, multiple vaccine-like-type-2 poliovirus isolates were collected from the London Beckton Sewage Treatment Works. Beckton is the largest sewage treatment works in Europe, serving a population of almost 4 million Londoners. Crucially, testing revealed that the isolates are genetically related and indicate poliovirus transmission across separate networks of individuals.

It is thought that the incident may have been triggered by an individual who had recently received an OPV overseas and was subsequently shedding viral material in their faeces when they arrived in the UK.

There have not been any reported cases of paralysis caused by poliovirus in the UK following the identification of vaccine-like-type-2 poliovirus isolates in London sewage.

Poliovirus has also recently been detected in New York and Jerusalem.

UKHSA response

The UK Health Security Agency alerted the NHS of the findings in June 2022 and reminded clinicians to report cases of paralysis and collect appropriate samples for testing. Owing to its status as a notifiable disease in the UK, medical practitioners must report suspected cases of polio to health protection teams.

UKHSA has also declared a national enhanced incident response to coordinate and respond to the incident, and the WHO has been notified of UKHSA’s findings.

Vaccination campaign

In response to the incident, effort has been made to improve polio vaccination coverage in London.

UKHSA has asked healthcare and school immunisation providers to ensure that all patients are up to date with polio vaccines, particularly in newly registered children and adults, migrants, asylum seekers and refugees.

A catch-up vaccination programme has been established for children under the age of 5, with plans to extend this to people up to the age of 18 years.

In addition, the Joint Committee on Vaccination and Immunisation has recommended that all children aged 1 to 9 in London should be offered a supplementary booster dose of the IPV.

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