Hard on the heels of the recent outbreak of serious hepatitis of unknown cause in children and the emergence of monkeypox, the UK Health Security Agency has issued a fresh alert concerning another unusual infectious threat: polio. No clinical cases of polio have been reported as yet but the worry has arisen because of routine testing of samples from the Beckton Sewage Treatment Works. The largest such facility in Europe, Beckton processes waste from around 4 million people living in the north and east of London. Since February, the same strain of poliovirus has been isolated on multiple occasions, implying there is a degree of community transmission occurring in the city.
As recently as the 1950s, polio outbreaks occurred regularly in Britain. Most people infected with any of the three strains of the “wild type” virus would have no symptoms. Some experienced a flu-like illness often accompanied by gastroenteritis, which would develop after an incubation period of up to three weeks. But the feared complication – occurring in 0.1 to 1 per cent of cases, depending on age of patient and strain of virus – was paralysis. In the worst outbreaks in the UK, during the early 1950s, as many as 8,000 people a year would be affected by the paralytic form of the disease.
Polio is most commonly transmitted by the faeco-oral route: large amounts of virus are shed in faeces and poor hand hygiene results in it being passed to others via the contamination of food. Less frequently, it can be spread through coughs and sneezes. However it gets there, poliovirus multiplies in the bowel of a new host, and has a high affinity for certain nerve tissues. In a minority of individuals, infection spreads to and kills the neurones that make muscles work, and paralysis ensues. This might affect, for example, a limb. The late Ian Dury, lead singer with 1970s band Ian Dury and the Blockheads, was left with a withered and weakened leg and hand as a result of the disease.
But if the muscles responsible for breathing are affected, polio can prove fatal. Images from the first half of the 20th century, of patients being nursed in “iron lungs” – coffin-like ventilators that assisted breathing – illustrate how serious this rare complication can be.
Polio was eradicated from Britain a generation ago as a result of a concerted vaccination campaign. The first preventative deployed in the late 1950s, inactivated poliovirus vaccine (IPV), used killed virus particles and was injected into muscle. It provoked a reasonable immune response. Even more efficacious, however, was the oral polio vaccine (OPV) brought into use in the early 1960s. Readers of a certain age will recall being given this on a sugar lump as children. OPV contains a weakened, or attenuated, live poliovirus, incapable of causing paralysis but which still replicates in the gastrointestinal tract for some weeks. The resultant immunity is far more vigorous. Additionally, the excretion of attenuated poliovirus for several weeks following vaccination is thought to provide useful booster immunity among members of the same household.
The problem with OPV is that, very occasionally, the attenuated virus mutates and regains the ability to cause paralysis – vaccine-derived poliovirus (VDPV). If a country has endemic wild type virus, the greater efficacy of OPV over IPV strongly favours the oral vaccine’s use – VDPV causes paralysis around 2,000 times less frequently than wild type poliovirus. But once a country has eradicated wild type polio, the balance of risk-benefit shifts. IPV, which never causes clinical disease, becomes the safer choice. Following the UK being declared polio-free in 2003, immunisation was switched to IPV in 2004. Injections are given in a three-dose regime in the first few months of life, and boosted prior to starting school and again as a young teenager.
Given that we have no wild type polio, and that we vaccinate exclusively with IPV, we should in theory never see poliovirus in sewage samples. But every year, a handful of isolates are found. These are invariably of the attenuated poliovirus used in OPV, and originate from travellers to or from places like Pakistan or Afghanistan, who have recently been vaccinated with OPV because wild type polio remains endemic in those countries. The attenuated poliovirus will usually have disappeared the next time a sewage sample is tested.
The situation at Beckton is different because the same virus has persisted over a period of several months, implying ongoing transmission. What has prompted the UK Health Security Agency to go public now is that the attenuated virus has mutated into VDPV, meaning it has acquired the capacity, in rare cases, to cause paralytic disease in the unvaccinated.
It couldn’t have happened in a worse location. While polio vaccine uptake in the UK is generally high, in London around one in ten young children are without protection, and as many as a third of teenagers remain unboosted. Efforts are now being made to contact those who are suboptimally vaccinated. Work to narrow down the areas where the virus is being transmitted is underway. Polio has reappeared in the context of the dent to routine vaccination provision caused by the pandemic, and in an era of heightened vaccine hesitancy fuelled by the controversies surrounding Covid immunisation. Aftershocks from the pandemic continue to reverberate, particularly in socio-economically deprived and minority ethnic populations.
Paediatric hepatitis. Monkeypox. Now polio. To unforgivably misquote Oscar Wilde: to experience one unusual potentially infective disease may be regarded as a misfortune; to suffer two might look like coincidence; a third appearing must raise questions about Covid. There is now incontrovertible evidence that infection with Sars-CoV-2 results in ongoing immunological dysfunction in a proportion of affected individuals. Is this latest health alert another piece in the puzzle of what it means, as a society, to be “living with Covid”?