Washington: An unusually deadly outbreak of polio in 2010 in the Democratic Republic of Congo was linked on Monday to a mutated form of the virus which common vaccines may not prevent.
The outbreak of polio infected 445 people and killed 47 percent of them, a rate much higher than a six percent death rate seen during an outbreak in Tajikistan the same year.
Researchers found that in 29 percent of cases studied, people who had been vaccinated against polio in the past did not produce the necessary antibodies to fight off infection.
The risk of such polio outbreaks in the future "is very hard to project," said the study in the Proceedings of the National Academy of Sciences, a peer-reviewed US journal.
The researchers said that in areas where not everyone gets vaccinated against polio, conditions are ripe for the emergence of "variant viruses whose fitness is normally impaired in populations with robust immunity."
Therefore, researchers recommend that people continue to be vaccinated against polio for years to come, potentially with both the inactivated and active vaccine types.
Polio is a potentially fatal viral disease that half a century ago killed or crippled hundreds of thousands of people, mainly children.
In 1988, the disease was endemic in 125 countries, and 350,000 cases were recorded worldwide, according to the World Health Organization.
Today, the virus is considered endemic in only three countries -- Afghanistan, Nigeria and Pakistan -- where the vaccination campaign has been attacked by Islamists and tribal leaders.