From VOA Learning English, this is the Health Report.

　　Is a new deadly polio virus hiding somewhere in the world? Some scientists believe this might be the case.

　　Polio does not usually kill. The disease can cause paralysis, leaving victims unable to walk or move parts of their bodies.

　　But polio was deadly during a 2010 spread of the virus in the Republic of Congo. Nearly half of the 445 people infected with the virus died.

　　VOA's Steve Baragona reports that new research says there might have been a weak spot in the polio vaccine. A mutated, or changed, polio virus was able to resist the antibodies created by the vaccine. The study says a mutated polio virus was responsible for that unusually deadly outbreak.

　　The researchers of the study say their findings show new and dangerous kinds, or strains, of polio may emerge as the goal of getting rid of the virus nears.

　　Scientist Felix Drexler studies viruses at the University of Bonn in Germany. He says the patients in the Congo outbreak were unusual in an important way.

　　"About half of them remembered having taken three doses of live vaccine. That made it even more bizarre, because if they had been vaccinated, they shouldn't be sick."

　　Mr. Drexler and researchers in Europe and Africa studied that polio virus. They found it had some new mutations. The mutations prevented antibodies that fight the virus from attaching to the virus.

　　"We thought, ‘Wow, maybe that could affect the ability of the antibodies in human blood to neutralize the virus.' And yes, it did."

　　The researchers reported their findings in the publication, Proceedings of the National Academy of Sciences. They tested the virus in Germany against blood samples from people with better than average vaccination coverage.

　　"Up to 30 percent actually had to be considered completely unprotected."

　　The Republic of Congo had been polio free before the 2010 outbreak. It took four nationwide immunization campaigns to stop the mutated virus.

　　Felix Drexler says the effort worked because almost every man, woman and child got vaccinated with the strongest form of the vaccine.

　　Historically, the polio vaccine has worked. But Mr. Drexler and others are now wondering how well it works.

　　"The question that the experts are asking is, is the vaccine good enough to enable us to eradicate polio virus?"

　　Experts say the end of polio is near. There have been fewer than 150 cases in the world this year. And the virus is found mostly in just three countries.

　　Olen Kew studies viruses with the U.S. Centers for Disease Control and Prevention. He says the problem in Congo was not the vaccine. He notes that the vaccine has prevented the polio virus everywhere it has been used. He blames Congo's civil unrest in the 1990s and early 2000s that had messed up vaccination campaigns.

　　"What happened in Congo was, it hadn't been used for quite a long period of time and a susceptible group opened up. And then when the virus was introduced, it had devastating effects."

　　Walt Orenstein is with the Emory Vaccine Center. He says stronger vaccines might be helpful. But, he adds, the end of polio is very close. He believes that the tools currently in use could do the job.

　　"I think the most important message, to me, is we need to push hard and push fast and terminate transmission as quickly as possible."

　　Mr. Orenstein says that health workers need to clear the last few areas where the polio virus exists. Then, he says, talk about mutant strains or stronger vaccines would be unnecessary.

　　I'm Anna Matteo.

　　Steve Baragona wrote this report and Anna Matteo adapted and produced it for Learning English.