Vaccine-Resistant Polio Strain Caused the Republic of Congo Outbreak

Poliomyelitis, more commonly known as polio, was once a major issue in the United States. It can cause muscle weakness and an inability to move that persists years after the person has recovered from the infection. That's why the polio vaccine was such a triumph; routine vaccination has helped reduce the number of cases by more than 99 percent in 30 years. Now, though, scientists may have discovered new mutations of polio that are resistant to the vaccine.

In 1988, there were 350,000 reported cases of polio. That number drastically dropped to just 650 in 2011. Yet major epidemics of polio still occur today, such as the outbreak in 2010 in the Republic of Congo. In fact, this particular outbreak resulted in an exceptionally high mortality rate of 47 percent; out of 445 confirmed cases, nearly 210 died.

The reason behind this outbreak was new mutations of polio. Scientists recently identified the virus responsible for the outbreak and sequenced its genetic material. The genetic sequence showed two mutations of the proteins that form the "shell," also known as the capsid, of the virus. This mutation complicates the task for the antibodies produced by the immune system of the vaccinated patient since they can no longer recognize the viral strain.

In fact, the researchers found that the antibodies created by the vaccine were less effective against the Congo strain than against other strains of the poliovirus. About 15 to 30 percent of those vaccinated probably would not have been protected during the 2010 epidemic.

Currently, the global campaign to eradicate polio is entering its final phase. Yet this latest finding shows that variants of the virus could emerge among populations who are immunized with the vaccine. This means that better clinical and environmental monitoring may be needed in order to completely eradicate polio.

The findings are published in the journal Proceedings of the National Academy of Sciences.