A leading coronavirus vaccine trial spearheaded by Oxford University was put on ice after a British participant developed severe neurological symptoms, AstraZeneca has confirmed.
A spokesman for the pharmaceutical giant said the woman, who had received a dose of the experimental vaccination, reported symptoms consistent with transverse myelitis – a rare inflammation of the spinal cord.
"We don't know if it is (transverse myelitis)," spokesman Matthew Kent told reporters on Thursday.
"More tests are being done now as part of the follow-up."
The vaccine, developed by Oxford University in partnership with Astrazeneca, is being tested in thousands of people in Britain and the US, and in smaller study groups in South America.
On Tuesday, the drug-maker said its "standard review process triggered a pause to vaccination to allow review of safety data."
It did not provide any details other than to say a single participant had an "unexplained illness."
Mr Kent said an independent committee was reviewing the study's safety data before deciding if and when the research could continue.
The study was previously stopped in July for several days after a vaccinated participant reported neurological symptoms.
However, this turned out to be an undiagnosed case of multiple sclerosis that was unrelated to the vaccine.
Late last month, AstraZeneca began recruiting 30,000 people in the US for its largest study of the vaccine.
Meanwhile trials have entered the late stages in the UK, and smaller studies are being carried out in Brazil and South Africa.
Several other Covid-19 vaccine candidates are in development.
Dr Soumya Swaminathan, the World Health Organisation's chief scientist, said the UN health agency was not overly concerned by the pause in the Oxford and AstraZeneca vaccine trial, describing it as "a wake-up call" to the global community about the inevitable ups and downs of medical research.
Temporary holds of large medical studies are not unusual, and investigating any serious or unexpected reaction is a mandatory part of safety testing.
