Two Ukrainian children have been paralyzed by polio, in the first recorded outbreak of the virus in Europe since 2010. One is four years old, and the other only 10 months old. Both cases were confirmed on Sept. 2 by the World Health Organization (WHO), reports Reuters.

Polio is contagious and incurable, and risk of the virus spreading further across Ukraine is high, Hamid Jafari, the head of WHO’s Polio Eradication Initiative told Quartz. In 2014, only 50% of Ukrainian children had been vaccinated against polio. Neither of the two stricken children had been vaccinated.

However, the risk of polio spreading internationally from Ukraine is low, notes Jafari. Although the outbreak-affected area borders with Romania, Hungary, Slovakia and Poland, polio immunization rates in these countries are above 90%—making it unlikely that the virus could travel far.

According to the WHO, polio only causes paralysis in one of every 200 cases. So Ukraine’s two cases of paralysis are likely not the only cases of infection, explained Oliver Rosenbaum, a spokesperson for the Polio Eradication Initiative, to the BBC. ”That’s one of the big dangers of the disease, there are a lot of asymptomatic cases,” he said.

This news follows the recent announcement that Nigeria is on its way to be polio-free in 2017. While Ukraine’s outbreak is a setback in global efforts to eradicate the crippling disease, Jafari is optimistic. “With several rounds of mass immunization,” he said. “We should be able to stop it.”