The pharma giant said it is standard routine to allow a review of safety data to ensure the integrity of the trials.
“In large trials illnesses will happen by chance but must be independently reviewed to check this carefully,” a spokesperson for the drug company told STAT News.
“We are working to expedite the review of the single event to minimise any potential impact on the trial timeline.”
The late-stage clinical studies were conducted on 30,000 people across the US, South Africa, England, Russia and Brazil.
Oxford’s Professor Andrew Pollard said in August data could be put before regulators as early as this year but ensuring safety and efficacy is paramount.
Analysts at Liberum noted "there is clearly a risk the vaccine fails at this hurdle", while if the adverse effect does not relate to the candidate "the study could be up and running again within weeks".
Astra shares shed 1% to 8,284p on Wednesday at the opening bell.
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