AstraZeneca has paused late-stage clinical trials of AZD1222, an experimental COVID-19 vaccine being developed in partnership with the University of Oxford, after a study volunteer experienced an unexplained illness.

In a move described by the firm in a statement as voluntary and routine, the clinical hold will allow for a thorough investigation of safety data to ensure that the integrity of the trials is maintained.

“In large trials illnesses will happen by chance but must be independently reviewed to check this carefully,” the drug giant noted.

“We are working to expedite the review of the single event to minimise any potential impact on the trial timeline.”

AstraZeneca was in line to be one of the first drugmakers to reap late-stage data for a potential COVID-19 vaccine, but the clinical hold could add a significant delay to its development.

“The trial was expected to read out by November (as an upper estimate) and potentially in the next few weeks. However, any suspension to the trial is likely to lead to delays in these timelines,” said analysts at Shore Capital, as reported by the Financial Times.

The news comes just days after AZ announced the expansion of the vaccine’s development programme into a Phase III clinical trial in the US to assess its safety, efficacy and immunogenicity.

This followed interim results from the ongoing Phase I/II COV001 trial published in The Lancet in July, which showed that AZD1222 was tolerated and generated robust immune responses against the SARS-CoV-2 virus in all evaluated participants.