A front-runner Covid-19 vaccine trial developed by AstraZeneca and the University of Oxford paused its phase three study after a volunteer developed an unexplained illness, AstraZeneca announced on Tuesday.
The potential vaccine was viewed as a leading candidate in the global search for an immunisation against the coronavisus, dimming hopes of a possible year-end roll-out.
AstraZeneca said it voluntarily made the decision to pause the trial in order to conduct a safety review into the potential vaccine and the adverse symptoms developed by the participant.
"This is a routine action which has to happen whenever there is a potentially unexplained illness in one of the trials," the company said.
The details of the adverse reaction have not been released, but according to a New York Times report, a participant in the UK was found to have transverse myelitis, an inflammatory syndrome that affects the spinal cord and is often sparked by viral infections. The volunteer is expected to recover.
“In large trials, illnesses will happen by chance but must be independently reviewed," said AstraZeneca.
It is not clear for how long the study will be suspended, but the company said it is “working to expedite the review of the single event to minimise any potential impact on the trial timeline".
The joint vaccine effort by AstraZeneca and Oxford University was holding late-stage trials at dozens of sites across the US, in addition to trials in the UK, Brazil, India, Japan and South Africa. A trial was also planned in Russia.
The vaccine study, officially known as AZD1222, was looking to recruit more volunteers in the US, with the aim of enrolling 30,000 participants at 80 sites across the country.
The discovery of the adverse reaction has affected all other trials, with researchers now looking for similar cases and symptoms among other volunteers, reported health news website Stat News.