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Why is polio back in UK? Where the outbreak is, how the virus has spread and the other unanswered questions

Health experts believe a combination of factors, including the type of vaccine used in other countries and a fall in vaccine coverage here, has contributed to the first person-to-person transmission in the UK for the first time since 1984

The discovery of poliovirus in London sewage raises concerning questions about the return of an infectious disease that was eradicated in the UK decades ago.

While there have been no confirmed cases, experts at the UK Health Security Agency believe there has been some limited person-to-person transmission, due to the detection of the virus in samples over a period of four months.

But in the early stages of this likely outbreak, there are still many unanswered questions.

Why is this happening now?

The last confirmed case of polio in the UK was in 1984, and the country was officially declared polio-free in 2003. But the disease and the virus that causes it have not been eradicated worldwide. Health experts believe that a combination of factors have caused this episode – but they do not know for sure.

There are two different polio vaccines used worldwide: the oral polio vaccine (OPV), which consists of a live but safe version of the virus, and the inactivated polio vaccine (IPV), which is injected and does not contain live virus. Each vaccine has advantages and disadvantages.

The OPV is effective both at preventing disease and transmission of the polio virus, but it carries a risk of causing, through the shedding of live virus, a vaccine-derived polio strain, which is weaker than the original virus but can still cause disease and, in a very small number of cases, paralysis.

The IPV does not carry this risk, and it is close to 100 per cent effective against disease, but it does not stop transmission of the polio virus. The IPV is therefore better suited to countries where polio has been eradicated – as in the UK, where it has been in use for nearly 20 years.

In this case, UKHSA experts believe that someone who had been given the OPV vaccine abroad travelled to the UK and, through viral shedding, a vaccine-derived polio virus has developed and been transmitted to members of their family.

This is potentially a problem because vaccine coverage of IPV in the UK has fallen in the past five years, and is lower in London than the rest of the UK – as low as 71 per cent for 14-year-olds receiving their booster dose. Weaker vaccine coverage could allow polio to gain a foothold.

Where exactly is the outbreak?

This question has not been answered, because the truth is the UKHSA does not know exactly. Polio virus was detected, through routine surveillance by public health officials testing for Covid and other diseases, in samples of sewage in a treatment works in east London which covers a population of four million people north of the Thames.

The samples only contained poliovirus RNA, not human DNA which, by the time the sewage had been processed, would have been removed, so there is no way it could be traced – and in any case, this would raise all sorts of ethical and privacy issues if it could. Experts are now carrying out a more detailed investigation in six local areas in north and east London to try to isolate the outbreak.

They can take samples from sewerage mains in a localised area, but this would still cover several thousand homes. Once a localised area is identified, then street by street testing for polio can take place and the outbreak isolated.

Has the virus spread?

This question is one that is troubling scientists and public health officials. At the moment, there is educated guesswork that the likely outbreak is confined to a small number of individuals – this is based on the number of samples found between February and May, and the fact that no polio cases have been identified in GP surgeries.

The concern is that the virus may have emerged at the start of this year – several months ago. Polio can spread between individuals who may not necessarily experience symptoms, or, given many of the symptoms are similar to covid or flu – such as a high fever, nausea and aches – some cases may have been missed. And, as mentioned above, the IPV vaccine does not prevent transmission of the polio virus, so in theory this could be spreading beyond one household.

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