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A vaccine-derived version of poliovirus has repeatedly surfaced in London sewage over the past several months, suggesting there may be a cryptic or hidden spread among some unvaccinated people, UK health officials announced Wednesday.
No polio cases have been reported so far, nor any identified cases of paralysis. But sewage sampling in one London treatment plant has repeatedly detected closely related vaccine-derived polioviruses between February and May. This suggests "it is likely there has been some spread between closely-linked individuals in North and East London and that they are now shedding the type 2 poliovirus strain in their feces," the UK Health Security Agency (UKHSA) said.
Though the current situation raises alarm, the agency notes that it's otherwise common to see a small number of vaccine-like polioviruses pop up in sewage from time to time, usually from people who have recently been vaccinated out of the country. This is because many countries use oral polio vaccines that include weakened (attenuated) polioviruses, which can still replicate in the intestines and thus be present in stool. They can also spread to others via poor hygiene and sanitation (i.e., unwashed hands and food or water contaminated by sewage), which can become concerning amid poor vaccination rates.
How and why this happens
Briefly, there are two types of polio vaccines: the attenuated oral vaccines and inactivated vaccines. Many high-income countries that are considered polio-free—including the UK and the US—use the inactivated vaccines, which do not have viruses capable of replicating or spreading. These vaccines are highly effective at preventing paralytic polio, but they do not produce high levels of local immune responses in the gut. So, if a vaccinated person encounters wild poliovirus, the virus may still be able to replicate in their gut and spread. In areas affected by wild polio outbreaks, this means that the virus can continue spreading.
Oral polio vaccines, on the other hand, can not only prevent paralytic polio, they can also produce strong local immune responses in the gut that block the virus from replicating there, thus disrupting its spread. These vaccines can also be more than five times cheaper than the inactivated kind. For all of these reasons, oral polio vaccines are the predominant vaccines used in the long, drawn-out battle to eradicate wild polio. Currently, wild polio is still found in Afghanistan and Pakistan, and Malawi and Mozambique have recently reported single cases.
But, one of the downsides to oral polio vaccines is that vaccinated people can shed the attenuated vaccine virus in their stool for several weeks after vaccination. If this happens in a community with poor sanitation, hygiene, and low vaccination coverage, the vaccine virus can spread from person to person. Over time, as the vaccine virus spreads to more people, it can pick up mutations that make it more like wild-type polio, allowing it to regain the ability to cause disease and, in rare instances, paralysis in unvaccinated people. At this point, the mutated vaccine virus gets dubbed "vaccine-derived poliovirus" or VDPV. Recently, VDPV cases have been reported from several African countries and Israel.
A cautionary tale in London
A VDPV is what health officials are now reporting in London: They found vaccine-like polio virus starting in February—likely from someone who had recently traveled to a different country where oral polio vaccines are used—and, since then, the virus appears to have continued to evolve and is now classified as a vaccine-derived poliovirus type 2 (VDPV2).
"Vaccine-derived poliovirus is rare and the risk to the public overall is extremely low," Dr. Vanessa Saliba, consultant epidemiologist at UKHSA, said. But, "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 [doctor] to catch up or if unsure check your [vaccination records]. 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."
Health experts say that risk in London is exactly why strong childhood vaccination programs and uptake are critical everywhere, even in countries where vaccine-preventable diseases are thought of as bygones. To be clear, polio vaccines protect against both wild and vaccine-derived polio.
"Parents sometimes ask why, when diseases are uncommon in UK, or in the case of polio has been eliminated, do we continue to vaccinate against them," David Elliman, consultant pediatrician at Great Ormond Street Hospital in London, said in a statement. "The answer is that, although we are an island, we are not isolated from the rest of the world, which means diseases could be brought in from abroad. The finding of vaccine-derived polio virus in sewage proves the point."
The Global Polio Eradication Initiative, led by the World Health Organization, put the point more succinctly, saying in a Wednesday announcement: "Any form of poliovirus anywhere is a threat to children everywhere."
Polio outcomes
In the US, travelers occasionally bring polio into the country, which last happened in 1993. But, the last time a polio case originated in the US was in 1979. In the UK, the last wild polio case originating there occurred in 1984, and the country was declared polio-free in 2003.
Most people infected with poliovirus have no symptoms, but about a quarter will develop a flu-like illness that clears on its own, according to the Centers for Disease Control and Prevention. In a smaller proportion—estimated to be between 1- to 5-in-1,000—the virus attacks the central nervous system, leading to more severe symptoms, including tingling in legs and arms, meningitis, and paralysis.
The CDC estimates that about 1-in-200 people infected with poliovirus will develop paralysis. And about 2 percent to 10 percent of people with paralytic polio will die because the paralysis will affect their ability to breathe.
For anyone who survives the infection—whether it's mild or severe—an estimated 25 percent to 40 percent will develop post-polio syndrome, which can cause pain, weakness, and paralysis 15 to 40 years after the initial infection.
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