The World Health Organization reports two children in southwestern Ukraine have been diagnosed with polio for the first time in Europe since 2010.
The 10-month-old and four-year-old living in the Zakarpattya region have been paralyzed due to the virus.
WHO confirmed that Ukraine was particularly vulnerable because of a decrease in immunization supplies. Last year, Ukraine only immunized 50 percent of children against polio and other preventable diseases.
The two cases are vaccine-derived polio, which is rare and occurs when the strains of poliovirus in the oral vaccine genetically mutate.
The weakened vaccine-virus replicates in the intestine for a period of time developing immunity by building up antibodies. During this time, the vaccine-virus is excreted and can continue to circulate and spread in the immediate community.
The strain must circulate in a seriously under-immunized population for a period of at least 12 months for this type of polio to occur.
National health authorities are currently in discussion in order to plan and implement a response to the outbreak.
Polio, which is highly contagious and has no cure, can lead to paralysis, breathing problems, or death. The infectious disease affects the most vulnerable populations including those living in places with poor sanitation.
According to WHO, more than 10 million cases of polio have been prevented, and the disease has been reduced by more than 99 percent since 2000.