It was hailed as a breakthrough vaccine for dengue fever, a neglected tropical disease that kills about 20,000 people each year. Dengvaxia, made by Sanofi Pasteur, was being rolled out to more than 800,000 schoolchildren in the Philippines when reports began trickling in of vaccinated children falling seriously ill, and some dying.

The 2017 rollout was halted; the country’s health minister, along with other officials and six Sanofi employees now await trial on charges of “reckless imprudence resulting in homicide”.

All the accused deny the charges and the company insists the vaccine is safe. But there remains an unresolved scientific mystery at the heart of the Dengvaxia tragedy: in a small subset of people, can a vaccine make an infection worse rather than better?