Health officials have said a girl from Coventry who died shortly after receiving a cervical cancer vaccine had "serious underlying medical conditions".
Natalie Morton died in hospital after getting the vaccine at Blue Coat Church of England School.
It was initially believed the vaccine, Cervarix, was the "most likely" cause of death.
But Dr Caron Grainger, joint director of public health for NHS Coventry and Coventry City Council, confirmed the 14-year-old had a medical condition which "was likely to have caused death".
The NHS has quarantined the batch of Cervarix used in the case of the schoolgirl, and manufacturers GlaxoSmithKline will carry out testing on it.
But health officials have insisted the vaccine is safe. Director of Health at the International Policy Network think tank, Philip Stevens, said there is a one in a million chance in developing a life-threatening reaction to the vaccine.
He said: "The probability of a British woman developing cervical cancer over her lifetime is one in 136, or 0.74% - that's 7,400 times greater".