A 29-year-old British man made medical history after doctors found that the polio vaccine he received as an infant mutated into a virulent strain which lived in his gut for nearly 30 years. Although the man has no symptoms of the virus, he has been shedding a highly contagious form of the virus in his stool. In a recent study on the case, researchers called the discovery a "worrisome new development in the polio end game" and expressed concern that his case may not be isolated.

The unnamed British man received the mandatory full course of oral vaccinations of the weakened polio virus in the form of a sugar cube at five, seven, and 12 months, followed by a booster at seven. However, according to The Guardian, a non-related health condition suppressed his immune system and prevented his body from destroying the virus. As a result, the polio virus was able to live inside of his gut for his entire life and was present in a large quantity in his stool.  

Viral shedding refers to the presence of a virus in body secretions which hold the potential for disease transmission and transfer. Although this behavior has been often noted, most commonly in the case of the herpes virus, this case marks the longest period of viral shedding that doctors have ever observed.

The oral form of the polio vaccine the man received stopped being used in Britain in 2004, when the country switched to an injectable form of the vaccine. However, researchers worry that this case may not be isolated. LiveScience reported that there have been 73 cases of people with immune problems who had the polio virus replicate in their intestines for prolonged periods and it's possible that there may be others. Although the man poses no threat to those who are fully vaccinated against the virus, if another individual with a similar health condition lived in a country where vaccinations are not encouraged, it could be enough to start a polio outbreak.

"These measures are needed to be able to identify and manage the possible risks of [divergent] strains spreading and causing disease in patients and the general population," researchers wrote in a recent study on the case.

The Global Polio Eradication Initiative set hoped to have the polio virus completely eradicated from Earth by 2018. So far, the GPEI in partnership with the World Health Organization has managed to reduce polio rates by 99 percent. However, international crisis such as war and political unrest have made the ultimate goal of completely eradicating the virus difficult. Currently three strains of wild poliovirus exists in the world. Unfortunately, vaccinations have led to the evolution of mutated strains of the virus, an obstacle that must be addressed before the virus is completely eliminated.  

Source: Dunn G, Klapsa D, Wilton T, Stone L, Minor PD, Martin J. Twenty-Eight Years of Poliovirus Replication in an Immunodeficient Individual: Impact on the Global Polio Eradication Initiative. PLOS ONE. 2015.