Following a recent outbreak of serious hepatitis of unknown etiology in children and the appearance monkeypoxThe UK Health Safety Agency has issued a new warning about another unusual infectious threat: polio. No clinical cases of polio have yet been reported, but concern has arisen due to routine testing of samples from the Beckton sewage treatment plant. The largest of its kind in Europe, Beckton recycles the waste of some 4 million people living in north and east London. Since February, the same strain of poliovirus has been isolated several times, indicating that there is some degree of community transmission in the city.
Back in the 1950s, there were regular outbreaks of polio in the UK. Most people infected with any of the three “wild-type” strains of the virus do not have symptoms. Some had a flu-like illness, often accompanied by gastroenteritis, which developed after an incubation period of up to three weeks. But a dangerous complication occurring in 0.1-1% of cases, depending on the age of the patient and the strain of the virus, was paralysis. During the worst outbreaks in the UK in the early 1950s, the paralytic form of the disease infected up to 8,000 people a year.
Polio is most commonly transmitted by the fecal-oral route: large amounts of the virus are shed in feces, and poor hand hygiene results in it being passed to other people through food. Less commonly, it can be transmitted through coughing and sneezing. However it gets there, the poliovirus replicates in the gut of the new host and has a high affinity for certain nerve tissues. In a minority of people, the infection spreads to the neurons responsible for muscle function and kills them, resulting in paralysis. This can affect, for example, a limb. The late Ian Dury, lead singer of the 1970s band Ian Dury and the Blockheads, was left with a withered and weakened leg and arm as a result of illness.
But if the muscles responsible for breathing are affected, polio can be fatal. Pictures from the first half of the 20th century showing patients being treated in “iron lungs” – coffin-like ventilators that help breathe – show just how serious this rare complication can be.
Polio was eradicated from Britain a generation ago with a concerted vaccination campaign. The first preventive vaccine deployed in the late 1950s, the inactivated polio vaccine (IPV), used killed viral particles and injected into muscles. This provoked a reasonable immune response. Even more effective, however, was the oral polio vaccine (OPV), introduced in the early 1960s. Readers of a certain age will remember being given this on a sugar cube as a child. OPV contains a weakened, or attenuated, live poliovirus that cannot cause paralysis, but still replicates in the gastrointestinal tract for several weeks. The resulting immunity is much stronger. In addition, isolation of attenuated poliovirus within a few weeks of vaccination is thought to provide a beneficial booster of immunity among members of the same household.
The problem with OPV is that it is very rare for an attenuated virus to mutate and regain the ability to cause paralysis, the vaccine-derived poliovirus (VDPV). If there is an endemic wild-type virus in a country, the higher potency of OPV compared to IPV clearly favors the use of an oral vaccine—VDPV causes paralysis about 2,000 times less often than wild-type polio. But once a country eradicates wild-type polio, the balance between risk and benefit shifts. IPV, which never causes clinical disease, is becoming a safer choice. After the UK was declared polio free in 2003, immunization was switched to IPV in 2004. The injections are given as three doses in the first few months of life and are boosted before school entry and again during adolescence.
Given that we do not have wild-type polio and that we exclusively vaccinate with IPV, theoretically we should never detect poliovirus in sewage samples. But each year, several isolates are found. They are invariably related to the attenuated poliovirus used in OPV and originate from travelers arriving in or from places such as Pakistan or Afghanistan who have recently been vaccinated with OPV, as wild-type polio remains endemic in these countries. Attenuated poliovirus usually disappears the next time a wastewater sample is analyzed.
The situation in Beckton is different because the same virus persists for months, implying constant transmission. Which prompted the UK Health Safety Agency to release information that the attenuated virus had mutated into VDPV, meaning it had acquired the ability to cause paralytic illness in the unvaccinated on rare occasions.
It couldn’t have happened in a worse place. While polio vaccine use in the UK is generally high, in London about one in ten children are not protected and up to a third of teenagers are not vaccinated. Efforts are underway to reach out to those who are under-vaccinated. Work is underway to narrow the areas of transmission of the virus. Polio has reemerged in the context of the weakening of routine vaccinations caused by the pandemic and in an era of heightened vaccine mistrust fueled by the Covid immunization controversy. The effects of the pandemic continue to be felt, especially among the socioeconomically disadvantaged and ethnic minorities.
Children’s hepatitis. Smallpox monkeys. Now polio. It is inexcusable to misquote Oscar Wilde: to experience one unusual potentially infectious disease can be regarded as a misfortune; suffering together may seem like a coincidence; the third appearance should raise questions about Covid. There is now overwhelming evidence that infection with Sars-CoV-2 leads to permanent immunological dysfunction in some patients. Is this latest health warning another piece of the puzzle about what it means for society to “live with Covid”?