The vaccine, developed with the University of Oxford, has been described by the World Health Organisation as probably the world's leading candidate and the most advanced in terms of development. The suspension dims prospects for an early rollout amid reports that the United States was aiming for a fast-track approval before the November presidential election.
The British drugmaker said it voluntarily paused trials to allow an independent committee to review safety data, and it was working to expedite the review to minimise any potential impact on the trial timeline.
"This is a routine action which has to happen whenever there is a potentially unexplained illness in one of the trials," the company said in an emailed statement.
The nature of the illness was not disclosed, although the participant is expected to recover, according to Stat News, which first reported the suspension due to a "suspected serious adverse reaction".
The US Food and Drug Administration defines an adverse event as one in which evidence suggests a possible relationship to the drug being tested.
According to a New York Times report which cited a person familiar with the situation, a participant based in the United Kingdom was found to have transverse myelitis, an inflammatory syndrome that affects the spinal cord and is often sparked by viral infections.
Whether the illness was directly linked to AstraZeneca's vaccine remains unclear, the report said. AstraZeneca declined to comment on the report.