The World Health Organization reported Monday that an 8-year-old boy in Laos became paralyzed from the poliovirus in early September and died five days later.
The case is believed to be the first infection in the country since 1993 and is part of an alarming reemergence of polio around the world in recent months. Ukraine and Mali -- two other countries where the virus was thought to have been wiped out -- also reported new cases of polio.
WHO officials said that genetic sequence showed that the virus was derived from an oral vaccine and that it may have been circulating in Laos' Bolikhamxay Province for more than two years. The oral polio vaccine contains a weakened virus, and when a child is immunized the virus replicates in the intestine and is excreted by the individual. In places where there are sanitation issues, this virus can infect others in the community before dying out.
When a population is seriously under-immunized this virus can circulate for a long time and mutate into a form that can paralyze. The district where the child lived in Laos has chronically low immunization rates: 40 percent to 66 percent between 2009 and 2014 and 44 percent in 2015 to date for three doses of oral polio vaccine.
Before this most recent infection, Laos had been polio-free since 1993, when its last case of indigenous wild poliovirus was reported.
The WHO said that the country's ministry of health, in conjunction with the WHO, UNICEF and the U.S. Centers for Disease Control and Prevention, sent a team to the boy's village to collect stool samples and that they have activated an emergency immunization effort for the province and several adjacent provinces.
Health officials said the risk of the virus spreading from Laos to other countries was low given the relatively limited travel to and from the area where the infecttion was found and given the emergency immunization effort.
In September, the WHO said poliovirus had returned to Europe after a five-year reprieve, paralyzing a 4-year-old and 10-month-old in the Ukraine. The two cases were in the southwestern part of the country -- bordering Romania, Hungary, Slovakia and Poland -- and the strain responsible, vaccine-derived poliovirus type 1 or cVDPV1, may still be a threat to the region.
The same month, Africa saw its first polio infections in a year when a 19-month old child of Guinean nationality became paralyzed.
The WHO has emphasized that the risk of vaccine-derived polio infection "pales in significance" to the health benefits associated with oral polio virus. It says that every year hundreds of thousands of wild polio cases are prevented with vaccines.