What is polio, how do you catch it, how is it prevented and when can you be vaccinated as a child on the NHS?
Public health officials have declared a national incident because poliovirus has been detected in a number of sewage samples.
With the last case of wild polio registered in the UK almost 40 years ago in 1984, and the country declared virus free in 2003, health officials are now working to trace the source and ensure there's no resurgence of the infection that can cause serious illness and paralysis in people not fully immunised.
As NHS staff begin identifying young children and families who may have missed their chance to get immunised during the last two years of lockdowns, we take a look at the latest advice from public health teams.
What is polio?
Polio, says the NHS, is caused by a virus that spreads easily when an infected person coughs or sneezes. It can also be caught from food or water that's been in contact with the poo of someone who has the virus.
Because of stringent vaccination programmes polio has been eradicated in large parts of the world. The UK saw its last case of wild polio in 1984 - with the country declared polio free in 2003.
Most people who get polio do not have symptoms, but some can experience a mild flu-like illness such as a temperature, tiredness, headaches, sickness and muscle pain which lasts for around 10 days. Only in rare cases does polio cause paralysis, which often people recover from after weeks or months, however the situation can become life threatening if that paralysis affects muscles used for breathing.
How do you prevent polio?
The best way to prevent polio is with vaccinations, which are free on the NHS.
The polio vaccine is part of the NHS routine childhood vaccination schedule and is given when a child is eight, 12 and 16 weeks-old as part of the 6-in-1 vaccine, at three years and four months old as part of the 4-in-1 (DTaP/IPV) pre-school booster and again at 14-years-old as part of the 3-in-1 (Td/IPV) teenage booster.
The NHS says youngsters must have all of these vaccinations to be fully vaccinated against the virus.
You can also have a polio vaccination at any point in your life if you've never had one before, even if you're not travelling to a country where there might be a risk of catching it while patients who have had polio before should also get vaccinated as it protects against different types of the virus.
What has been found this week?
Samples of sewage water being routinely tested at the London Beckton Sewage Treatment Works have tested positive for vaccine-derived poliovirus. This is a type of the virus shed by someone who has recently been given the live oral polio vaccine OPV, which is administered in many countries abroad.
While it is normal, say health officials, to find one to three 'vaccine-like' polioviruses in sewage samples every year they are ordinarily one-off findings and never detected again.
However on this occasion several closely-related viruses have been found in a number of sewage samples taken between February and May, suggesting that has been some spread of the virus between closely-linked individuals who are now shedding that polio strain in their faeces. Vaccine derived poliovirus can cause serious illness, such as paralysis, in people not fully vaccinated should they come into contact with it.
So far, the virus has only been detected in the sewage samples and no associated cases of paralysis have been reported or detected among GPs or hospitals but investigations, say the UK Health Security Agency, will now try and establish if community transmission from person to person is happening.
Investigations are now underway to try and trace the source of this outbreak, and wastewater surveillance is also being expanded to try and determine the extent of any transmission, but people are being urged to ensure their vaccination records and those belonging to family members are up to date.
There is particular concern about small children who may have missed their routine polio vaccinations as babies and toddlers during lockdown, when many families stayed away from health services believing they were unavailable for immunisations.
The UK is considered by the World Health Organization to be polio-free, with low-risk for polio transmission due to the high level of vaccine coverage across the population.
However, vaccine coverage for childhood vaccines has overall decreased nationally and especially in parts of London over the past few years, not helped by the pandemic and lockdowns, so UKHSA is urging people to check they are up to date.
Overall however, officials wish to stress that at present, the risk to the public is remains very low.
Dr Vanessa Saliba, Consultant Epidemiologist at UKHSA said: "Vaccine-derived poliovirus is rare and the risk to the public overall is extremely low.
"Vaccine-derived poliovirus has the potential to spread, particularly in communities where vaccine uptake is lower. On rare occasions it can cause paralysis in people who are not fully vaccinated so if you or your child are not up to date with your polio vaccinations it’s important you contact your GP to catch up or if unsure check your Red Book. Most of the UK population will be protected from vaccination in childhood, but in some communities with low vaccine coverage, individuals may remain at risk."
Checking your child's vaccination status
Parents can check their child's vaccination status in their offspring's Red Book - given to every family when their child is born to record health information and important milestones.
Anyone who suspects that their child has missed a routine vaccination, regardless of their age, should contact their GP surgery for advice and to book an appointment.
The NHS is also to begin contacting parents of children aged under five, who they believe are not up to date with polio immunisations, starting with parts of north and east London closest to the treatment works in Newham which has identified the contaminated samples.