The World Health Organization says medical teams in Gaza are wrapping up the final day of an emergency polio vaccination campaign following the discovery of the territory’s first-known case of the illness in more than 25 years.

Dr. Rik Peeperkorn, the U.N. health agency’s representative, told reporters in a news conference from Gaza that the health workers had reached more than an estimated 552,000 children under the age of 5. They used a new oral polio vaccine targeting the specific type of polio seen in Gaza, which is a mutated strain that originated in an older oral vaccine.