Global advancements in the fight against polio have seen all but two countries declared free of the disease, with Pakistan and Afghanistan being the two remaining countries where the disease is still being circulated.

But an alarming new study describes the case of an anonymous male patient in the UK who has carried the virus in his body for almost 30 years. Researchers from the National Institute for Biological Standards and Control analysed more than 100 stool samples from the man collected over a period of 20 years and found each contained high levels of the polio virus (strain 2).

How could this be? After all, the man in question had received his full course of childhood immunisations, with an oral polio vaccine administered at five, seven, and 12 months of age, plus a booster when he was about seven years old.

According to the study, which is published in PLOS Pathogens, he was later diagnosed with common variable immunodeficiency, which means his immune system couldn't adequately kill off the small amount of the virus delivered to his digestive tract by the vaccinations.

But when doctors analysed his stool samples, they found that the strains of vaccine-derived polio virus within were different from the weakened vaccine strain with which he was immunised as a child. What this means is that the virus in his system had mutated in his body over the course of 28 years, developing into a potentially more dangerous form.

Fortunately, the researchers believe the excreted virus wouldn't threaten healthy people who have received polio vaccinations, but that doesn't mean we shouldn't be concerned about the risks of immunodeficient individuals.

"[O]f the total of 73 [similar] cases that have been described between 1962 and 2014, only seven involved infections lasting more than five years," write the researchers. "The case described here represents by far the longest period of excretion described from such a patient and the only identified individual known to be excreting highly evolved vaccine-derived poliovirus at present."

However, sewage samples collected in Slovakia, Finland, Estonia, and Israel have also revealed the presence of the polio virus, which suggests that 'chronic excreters' (ie. other immunodeficient individuals who pass the virus in their waste) are living in those countries and may pose an ongoing risk to eradication efforts.

In the worst-case scenario, the transmission of these strains to non-vaccinated individuals could even cause a new outbreak of the disease, leading the researchers to argue that we need enhanced surveillance systems to track any such strains and to develop anti-viral treatments to interrupt virus replication in immunodeficient individuals – which so far do not exist.

"The study has implications for the ecology of poliovirus in the human gut and highlights the risks that such vaccine-derived isolates pose for polio re-emergence in the post-eradication era," they write.