At least 15 children, all or most under age 2, have died after receiving an injection in a measles immunization campaign in an opposition-held area of northern Syria. Up to 50 more children were sickened.

Details are hazy, says a World Health Organization (WHO) representative in Geneva, Switzerland, but at this point the cause looks like a “very bad human error,” in which a strong muscle relaxant was administered instead of the measles vaccine. The tragic deaths threaten to undermine all vaccination efforts across Syria, where childhood immunization rates have dropped precipitously after years of civil war.

WHO and the United Nations Children's Fund (UNICEF) have dispatched an investigation team but for now are dependent on secondhand information from nongovernmental organizations and other partners in northern Syria, says WHO’s Christian Lindmeier. (For security reasons, neither organization has staff on the ground in Idlib, where the deaths occurred.) Until the cause is confirmed, rumors will continue to circulate, he warns; various press accounts are alleging a plot by the government of embattled Syrian President Bashar al-Assad or perhaps the terrorist group ISIS.

According to Lindmeier, the children died almost immediately on Tuesday after receiving the shot, part of a measles immunization campaign under way in Idlib and Deir al Zour, two governorates of Syria.

They were stricken with paralysis and respiratory distress, and most of the deaths were a result of suffocation. Up to 50 mostly older children up to age 6 were sickened with respiratory distress, diarrhea, and vomiting. WHO officials suspect the older children survived because the dose of the muscle relaxant, atracurium, is determined by weight.

The mistake appears to have happened in one of the hubs in Syria that distributes vaccines to health facilities. Apparently the vaccine was stored in a refrigerator along with other medicines, including atracurium, which is used in anesthesia. The measles vaccine is a freeze-dried powder, which is then mixed with a liquid dilutent, also supplied by the manufacturer, and refrigerated.

“How it could happen is completely unclear,” Lindmeier says, but “someone must have packaged the bags wrongly.”

Some 30,000 children had been vaccinated during the first part of the campaign in Deir al Zour and 12,000 in Idlib, with no adverse effects. After the deaths, the campaign was stopped almost immediately.

The immunization drive was being conducted by local health authorities with vaccine supplied by WHO and UNICEF to Turkey, which then delivered it across the border. The circuitous route is used because the Syrian government’s vaccination campaigns do not usually reach the rebel-held areas.

It is “very unlikely” that the vaccine was intentionally spiked, Lindmeier says. But we have to find out exactly what happened, he says. “If we are not 100% sure there will be theories.

Whatever the cause, the incident will be a major setback to vaccination efforts in Syria. Says Lindmeier: “It will be a huge challenge to restore trust in the vaccination campaigns.”

*Update, 19 September, 11:35 a.m.: The number of deaths in this story has been updated from 16 to at least 15 to reflect the number of deaths described in a joint WHO-UNICEF statement on 17 September.