Polio has been eradicated in but two countries in the world but a discovery that large amounts of live polio virus had thrived inside a man for more than 20 years indicates that threats still exist and can complicate eradication efforts.
There are two types of polio vaccines and three strains of wild polio virus. Inactivated polio vaccine (IPV) has been proven effective and safe in stimulating neutralizing antibodies but it can't induce mucosal immunity which will prevent the virus from being excreted. Oral polio vaccine (OPV), on the other hand, can neutralize antibodies while inducing mucosal immunity, killing polio viruses within the gastrointestinal tract and reducing levels of excretions.
The problem with excreted viruses is that they lead to circulating vaccine-derived polioviruses (cVDPV). More than 90 percent of cVDPVs are from the strain 2 polio virus, which has not been observed in the general population since 1999. As such, the strain would appear to have been eradicated. To help stop the spread of cVDPVs, the World Health Organization is planning on using OPVs that contain only the strains 1 and 3. When polio is finally eradicated, OPVs will be discontinued and only IPV immunizations will be utilized to bolster protection from the virus as needed.
In a study published in PLOS Pathogens, Javier Martin and colleagues examined over 100 samples of stool from a white male collected from 1995 to 2015. The man had received all his shots as a child, including OPV at 5, 7 and 12 months and a booster vaccine at 7 years. He was diagnosed later on with an immunodeficiency, which made his immune system incapable of killing viruses in his gut.
However, the polio virus present in all of the samples were not the same as the weakened strain 2 type, diverging from the original virus around 28 years ago when the man received his last OPV shot. All polio virus in the man's body had mutated, resulting in a different version of the strain 2 virus.
From 1962 to 2014, there were 73 cases of iVDPV or the presence of vaccine-derived polio virus from immune-deficient individuals. The man who was the subject of Martin's study though represented the longest recorded period of viral secretions.
The researchers concluded that developing efficient anti-viral treatments and enhanced surveillance will aid in interrupting the polio virus from replicating in immune-deficient individuals to manage cases of iVDPV. Additionally, new polio vaccines based non-infectious particles similar to viruses may help.
Photo: CDC Global | Flickr
