LONDON (AP) — Global health officials say they have identified two cases of polio in Mozambique caused by a mutated virus in the vaccine, marking another setback for attempts to eradicate the crippling disease.

In a report this week, the World Health Organization and partners said they had confirmed two polio cases in a 6-year-old girl and one of her contacts. Officials said isolates tested found they were infected by a virus derived from the vaccine. In rare cases, the virus in the oral polio vaccine can mutate into a form capable of causing new outbreaks.

Officials have missed repeated deadlines to wipe out polio and are facing numerous challenges in the remaining countries where the disease circulates. Last year there were dozens of cases elsewhere in Africa including Nigeria, Congo and Somalia.