By Reuters Staff

GENEVA/LONDON (Reuters) - Laos has suffered a case of vaccine-derived polio, the World Health Organization said on Monday, in a new setback to a global plan to eradicate the crippling disease after the virus resurfaced in Ukraine and Mali.

The WHO said an 8-year-old boy died of the disease on Sept. 11, and genetic sequencing suggested the virus strain has been circulating in the area of Bolikhamxay province, which has low immunization rates, for more than two years.

A global vaccination campaign has all but beaten the wild polio virus, with only Pakistan and Afghanistan reporting cases of wild polio virus infection this year.

Specialists have warned that vaccine-derived cases - such as this one in Laos and previous ones in Ukraine and Mali - could hamper progress towards global eradication.

The WHO stressed that "ending polio for good requires eliminating both wild and vaccine-derived polio".

It added, however, that because of relatively limited travel to and from this area, and because of extra immunization campaigns planned in response, the risk of international spread of this polio strain from Laos is low.

Laos has been free of the wild polio virus since 1993, but poor immunization rates mean people are at risk of infection with strains of the virus that can mutate in sewage after being excreted by immunized children.

The risk of vaccine-derived polio cases can be avoided by switching from using live oral polio vaccines (OPV) - which are highly effective, cheap, easy to deliver but contain live virus, - to "inactivated" vaccines (IPV), which are not effective for fighting endemic disease but contain no live virus.

The WHO said the use of OPV is being scaled down in a phased manner as countries eliminate circulating wild polio virus strains.