A UN humanitarian aid adviser for Syria says trucks are being prepared to ship polio vaccine into Islamic State group-held areas of Deir Ezzor governorate following confirmation of a “very dangerous” outbreak of the virus, The Associated Press reports.

Jan Egeland made the comments to reporters Thursday, June 15 in the wake of the confirmation announced last week by the World Health Organization of two cases in Deir Ezzor of a polio strain derived from vaccines that mutated under weakening health and immunization conditions.

He said 58 acute flaccid paralysis cases, a possible symptom of polio, had been reported in Deir Ezzor this year through June 6.

Egeland said “it’s one of the remarkable things” of Syria’s war that people have been reached with vaccines, even in the Deir Ezzor and Raqqa governorates held by IS.