UK health officials are insisting that a cervical cancer vaccine is safe after a 14-year-old who died shortly after being given the jab was found to have had an underlying medical condition.
However, the preliminary post-mortem examination findings on Natalie Morton did not completely rule out the possibility of a link between her death and the injection.
While the NHS said it was 'most unlikely' the HPV vaccination caused her death, questions remain about the nature of her medical condition.
The British government said it had confidence in the cervical vaccination programme and urged parents and schools use it.
Ms Morton collapsed at the Blue Coat Church of England School in Coventry, England on Monday and died in hospital.
Dr Caron Grainger of the UK National Health Service said last night: 'The preliminary post-mortem results have revealed a serious underlying medical condition which was likely to have caused death.
'We are awaiting further test results which will take some time.
'However, indications are that it was most unlikely that the HPV vaccination was the cause of death.'
The NHS initially quarantined the batch of Cervarix vaccine, while schools and primary care trusts temporarily suspended the immunisation programme.
Manufacturer GlaxoSmithKline recalled the specific batch of vaccine for further testing.
'As a purely precautionary measure we have asked the NHS to quarantine all stocks of HPV vaccine from the batch related to this case,' a UK Department of Health spokesman said.
The NHS started the national HPV immunisation programme in September last year, offering vaccines to girls aged 12 and 13 and to 17 and 18-year-old girls.
Vaccination is not compulsory and consent is required before it is administered to under-16s.