LONDON (AP) — Health officials in Burundi have declared an outbreak of polio linked to the vaccine, the first time the paralytic disease has been detected in the East African country for more than three decades.

Polio has been diagnosed in an unvaccinated four-year-old child in the western part of the country and in two other children who were contacts of the child, authorities in Burundi confirmed in a statement on Friday. Officials also found traces of the virus in sewage samples, confirming the circulation of polio.

The virus that sickened the children was found to be a mutated strain of polio that initially came from an oral vaccine.

The Burundi government declared the polio outbreak to be a national public health emergency and plans to start an immunization campaign within weeks, aimed at protecting all children up to age seven.

“We are supporting the national efforts to ramp up polio vaccination to ensure no child is missed and faces polio's debilitating impact,” said Dr. Matshidiso Moeti, WHO's Africa director.

The epidemic is another setback for the global effort to wipe out polio led by the World Health Organization and partners, which first began in 1988 and initially aimed to eradicate the disease in a dozen years.

Polio is a highly infectious disease mostly spread through water and typically strikes children under five. There is no treatment. Although the oral vaccine used in the global effort to eradicate the disease is highly effective, it requires four doses.

The oral vaccine can also cause polio in about two to four children per 2 million doses. In extremely rare cases, the weakened virus can also sometimes mutate into a more dangerous form and spark outbreaks, especially in places with poor sanitation and low vaccination levels.

In recent years, the oral polio vaccine has caused far more cases of polio than the wild polio virus. Last year, cases linked to the oral vaccine turned up in rich countries including Britain, Israel and the U.S. for the first time in years.

Officials began rolling out a new oral polio vaccine last year that they hoped would be less likely to mutate into a version able to trigger new outbreaks. But the epidemic in Burundi — in addition to six cases in Congo — were found to have been sparked by the new oral vaccine.

Across Africa, more than 400 cases of polio last year were linked to the oral vaccine, including Congo, Nigeria, Ethiopia and Zambia.

The disease also remains stubbornly entrenched in Pakistan and Afghanistan, where transmission has never been stopped.