In 2010 the fight to eradicate Polio took a hit with a large outbreak in the Republic of the Congo. Almost 450 cases were confirmed in the laboratory, but the large number of effected individuals wasn't even the most alarming part of this outbreak. What struck healthcare workers the most was the unusually high fatality rate: about 47%. The wild type strain isolated from this outbreak was one never before seen and had a unique mutation on a critical antigenic site.
This mutation played out in a very alarming way—mainly that this mutant virus appeared to be able to evade the immune system, even of those who had been vaccinated. In further studies on this virus, it was found that about 20% of vaccinated patients were not protected against this strain, even though their immunity against Sabin-1 remained intact.
Based on it’s genetic structure, it appears to have derived from a Southeast Asia strain, although the unique antigenic mutation is not found in this parent strain. However, this analysis raises important questions about how to combat potential vaccine-resistant strains—and what we should do if those strains come from the live attenuated virus itself.
Read the Paper here: http://www.pnas.org/content/111/35/12889