An investigation into the deaths of 15 children inoculated against measles in the northwestern province of Idlib has been launched by Syria's opposition.
The vaccination programme was carried out in areas under rebel control near the Turkish border but halted on Tuesday after reports of the deaths.
Some 100 other children are believed to have been affected and suffered reactions.
The opposition said it had set up a team bringing together health and justice officials to establish "the circumstances of this human tragedy."
In a statement, the opposition justice ministry announced the creation of a commission of inquiry "aimed at establishing the circumstances around the accident, to determine the real causes... and to take the necessary judicial procedures."
The Syrian Observatory for Human Rights, a monitoring group, said medical sources had suggested that the vaccines could have been compromised, either because they were expired or poorly stored, but there was no immediate confirmation of their claims.
The opposition's health ministry said the affected children started showing symptoms "half an hour after innoculation. They suffered diarrhoea, allergic reactions and breathing difficulties."
"The vaccines are the same as the ones used everywhere in the world," it said, adding that the expiry date on the vaccines was January 2016.
Syria's war has caused more than half of the population to flee their homes and millions of children are among the displaced, both inside and outside the country.
Medical groups have rushed to head off the spread of measles, mumps, rubella and polio in Syria, where normal medical services have disintegrated because of the civil war, which erupted in 2011.