LONDON (AP) — The World Health Organization says officials have found two children stricken by polio in Ukraine, the country's first cases of the paralytic disease in nine years.

Health officials had warned Ukraine was at high risk of a polio outbreak due to its low vaccination rates; only half of children were immunized against diseases like polio last year.

In a statement this week, WHO confirmed polio in two children in southwestern Ukraine. The cases resulted from a mutated polio virus in the vaccine. In rare instances, the virus in the vaccine can evolve into a strain that causes new outbreaks.

WHO said the risk of Ukraine exporting polio to other countries was low but noted the region where the cases were found shares borders with Romania, Hungary, Slovakia and Poland.