Researchers identified a vaccine-derived mutated form of polio virus in all 100 feces samples from a man who was vaccinated against polio in childhood.
Polio, or poliomyelitis, is a highly infectious disease caused by the polio virus, which invades the brain and spinal cord. The virus is most commonly spread through contact with the feces of an infected individual, though it can also be transmitted through cough or sneeze droplets.
While many individuals with polio experience flu-like symptoms such as sore throat, nausea and headache, more severe cases can lead to paralysis. Among those who become paralyzed, 5-10% die due to breathing problems.
Thanks to the two available polio vaccines - inactivated polio vaccine (IPV) and oral polio vaccine (OPV) - the disease has been eliminated in most countries. The only countries where polio remains endemic are Afghanistan, Pakistan and Nigeria - though the latter has not seen a case for over a year.
As such, the World Health Organization (WHO) are confident that polio can be completely eradicated in the near future. But this latest discovery - details of which are published in PLOS Pathogens - could hamper such hopes.
Weakened live virus in OPV vaccine mutated to become more deadly
For their study, co-author Javier Martin, of the National Institute for Biological Standards and Control in the UK, and colleagues assessed more than 100 stool samples from one white British male between 1996 and 2015.
Fast facts about polio
- Polio most commonly affects children under the age of 5 years
- The number of polio cases worldwide has fallen by 99% since 1988
- Around 1 in 200 cases of polio lead to irreversible paralysis.
The man had received a full course of polio immunization, including three doses of OPV - consisting of a weakened live poliovirus - at the ages of 5, 7 and 12 months and a booster vaccine at the age of 7 years.
Shortly after, the man was diagnosed with immunodeficiency, which the researchers note can impair the immune system's ability to destroy viruses in the digestive tract.
The team identified high levels of strain 2 poliovirus in all of the man's stool samples, which is a highly virulent strain that causes paralysis.
Further investigation revealed that the virus excreted in the samples was different to the weakened vaccine strain, indicating that the virus had mutated to become more dangerous. The team believes he has been shedding it in his feces for the past 28 years.
With the OPV vaccine, the gut normally clears the weakened polio virus within 6-8 weeks, though immune system disorders may stop this from happening, which is what Martin and colleagues believe has occurred in this case.
The researchers say that while high immunization coverage is likely to protect against paralytic disease caused by polio, their findings suggest "significant changes in immunization strategies" may be needed to prevent the spread of mutated polio virus and reach the goal of total polio elimination.
"New polio vaccines such as those based on noninfectious virus-like particles or even new genetically designed stable live-attenuated versions with no associated risk of producing VDPVs [vaccine-derived polio viruses], might be required to complete polio eradication."
The team says the development of such vaccines may be of great importance, particularly after mutated strains of polio virus were recently identified in sewage samples from Finland, Estonia, Israel and Slovakia.
"These VDPV isolates showed molecular properties typical of iVDPVs [vaccine-derived viruses from people who are immunodeficient] described above indicating that an unknown number of these chronic excreters exist elsewhere," they note.
In June, Medical News Today reported on a study published in PLOS Biology suggesting the polio virus can be spread for long periods of time without any cases occurring - another factor that researchers say could stand in the way of polio eradication.