[BEIJING] China's first polio outbreak in more than a decade was caused by a virus derived from the live but weakened virus used as a polio vaccine, say scientists.
It was the world's fifth outbreak of vaccine-derived polio since 2000.
The researchers say this highlights the need for policymakers to consider risks from vaccine-derived viruses when devising national immunisation plans.
The team was led by Yu Jingjin, deputy director of the Department of Disease Control at the Chinese Ministry of Health. The findings were published online in the Journal of Infectious Diseases last week.
Polio was once common worldwide but it has almost been eradicated thanks to widespread vaccination since the 1960s.
China was certified polio free in 2000. But in 2004, scientists identified six children in Guizhou, China's poorest province, with a sudden and lasting paralysis that resembled the effects of polio.
Yu's study showed that children had been infected by a virus genetically-similar to the one used in the oral vaccine.
Live vaccines are made from viruses that have been weakened so that they can no longer cause disease. The study suggests that the vaccine-virus mutated to regain virulence much faster than was previously thought possible.
Walter Dowdle, of the Task Force for Child Survival and Development, and Olen Kew, of the US Centres for Disease Control and Prevention, comment on the findings in an editorial in the same journal.
They say the outbreak shows that, in countries with high overall immunisation levels, vaccine-derived poliovirus can emerge in areas where immunisation is sporadic.
Guizhou has the lowest polio-immunisation rate of China's provinces 72 per cent compared to the national average of 90 per cent. None of the infected children had been immunised.
After the discovery, all children in the province under five years old were vaccinated and since then no cases of polio-like paralysis have been reported.