An Afghan health worker administers the polio vaccine to a child during a vaccination campaign on the outskirts of Jalalabad on Aug. 17. (Noorullah Shirzada/AFP/Getty Images)

Poliovirus has returned to Europe after a five year reprieve, paralyzing a 4-year-old and 10-month-old in the Ukraine, the World Health Organization said Wednesday.

The WHO said the two cases were in the southwestern part of the country -- bordering Romania, Hungary, Slovakia and Poland -- and that the strain responsible, vaccine-derived poliovirus type 1 or cVDPV1, may still be a threat to the region.

cVDPV is a rare, mutated form of the virus that comes from the vaccine itself. Oral polio vaccines contain a weakened form of the virus that activates an immune response in the body so that it builds up antibodies to protect itself. But it takes some time for this to happen, and meanwhile the virus replicates in the intestines and can be excreted by the person immunized and can spread to others in the community.

This isn't always a bad thing. It can sometimes provide what is called passive immunity to those who come into contact with it.

However, in populations that are under-immunized -- the Ukraine is definitely one of those with only 50 percent of children fully immunized against polio and other vaccine-preventable diseases in 2014 -- the vaccine-virus can circulate for long time, 12 months or longer, and genetically change into a more virulent form that can paralyze. This is what WHO believes happened in the Ukraine.

In a statement, WHO officials said that discussions are "ongoing" with Ukranian health authorities to implement "an urgent outbreak response." This type of plan usually involves a vaccination campaign to cover at least 2 million children under age 5.

"[T]he risk of further spread of this strain within the country is deemed to be high," the WHO said.

Polio disappeared for many years in the developed world, but it still exists in along the Afghanistan-Pakistan border. There were outbreaks in Syria and Iraq in recent years but not since 2014. The WHO, in cooperation with the U.S. Centers for Disease Control, the United Nations Children's Fund (UNICEF), Rotary International, and the Bill and Melinda Gates Foundation, have mounted an aggressive effort in recent years to eradicate polio from the world -- saying the goal is within reach with better rates of vaccination.

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