ON JUNE 8th reports of a suspected case of polio came from Venezuela. Fortunately, it turned out to be a false alarm. The report that came from Papua New Guinea on June 22nd, though, is no fiction. It was issued by the World Health Organisation and concerns not one, but three children who have tested positive for a threatening polio virus.

Around the world, polio is in full retreat. A mere three countries are still known to harbour wild polio viruses. These are Afghanistan, Nigeria and Pakistan. In 2017 only 22 cases of polio caused by such wild viruses came to the attention of the authorities. Unfortunately, the reason for this success, which is the extensive vaccination against polio of children throughout the world, can occasionally backfire and itself cause polio outbreaks.

In many countries polio vaccine includes live, attenuated viruses which breed in the recipient’s intestines and then enter the bloodstream, thereby triggering a protective immune response. An attenuated virus is one that has been weakened to a form which is not hazardous to health but remains potent enough to provoke the immune system into providing lifelong cover against real infection. Occasionally, though (less than once in every 17m vaccinations), the replication of such an attenuated virus throws up a mutation which creates a new strain. And because the vaccine-virus is present in someone’s faeces for six to eight weeks after inoculation, this new vaccine-derived polio virus (VDPV) can be released into the environment.

When such cases arise and the population is under-immunised (meaning that there are many susceptible children in it) then the vaccine-derived virus can circulate and, over the course of a year or so, reacquire virulence though additional mutations. That means it has the capacity to cause disease. These new strains are called circulating vaccine-derived polio viruses and have, in the past, caused outbreaks of polio in places such as Syria and Congo.

They are also culpable in Papua New Guinea. In April a six-year-old boy with paralysis, the most serious symptom of polio, but one that can have other causes, was confirmed to have a VDPV. Since then, two other children, who are not paralysed, have been found to be carrying the same VDPV, which is thus now categorised as a circulating virus.

The usual way for polio to spread is by people drinking water contaminated with faeces containing the virus. This seems to be what has happened in Papua New Guinea. Less than half of the country’s population has access to clean water and 81% do not use proper toilets. Moreover, in Morobe province, where the three children involved live, just 61% of children have received the recommended course of vaccinations. Perfect vaccination coverage in any country is rare, but leaving more than a third of the population unprotected is dangerous. At least 80-85% must be fully immunised to ensure that polio cannot spread among unvaccinated individuals if it is ever reintroduced.

A coalition of international health organisations therefore descended on Morobe province when the first case was confirmed, to conduct immunisations and improve surveillance in order to contain the spread of the virus. These groups are right to be vigilant. The longer a vaccine-derived polio virus is permitted to circulate unabated, the more people it can infect and the more time it has to evolve into a properly adapted pathogen.

The rapid reaction to the situation in Morobe province means this outbreak might soon be contained, with few additional instances of paralysis. But for a preventable disease with no cure and crippling symptoms, even a handful of cases is serious. And there are many other places in the world where a lack of proper sewerage and inadequate vaccination might allow something similar to happen. Sanitation and hygiene, unglamorous though they are, rank highly among development goals for good reason.