A phase I/II clinical trial for AstraZeneca’s Oxford University-partnered COVID-19 vaccine AZD1222 has resumed in Japan, following the restart of trials in the UK, Brazil, South Africa and India.

Earlier this month, clinical trials resumed in the UK after a standard review process was triggered when a participant in the British trial experienced a suspected serious adverse reaction.

According to the New York Times, the UK volunteer had been diagnosed with transverse myelitis, an inflammatory syndrome that affects the spinal cords and is associated with viral infection.

Although many countries have deemed that the clinical trials of AZD1222 are safe to continue, regulators in the US have still not resumed trials following the reported side-effect.

AZ said that it is continuing to work with the US Food and Drug Administration (FDA) to facilitate a review of the information needed to help it make a decision with regards to resuming the US trial.

“The safety of trial participants is of paramount importance and we are committed to upholding the highest standards of conduct in clinical trials,” the company said in a statement.

However, a report from Reuters detailed that the FDA has broadened its investigation into the serious adverse event within the AZ vaccine trial.

Three sources familiar with the matter told Reuters that the agency will consider data from earlier trials of similar vaccines developed by the same scientists before making a decision.

This is likely to lead to more delays for AZ’s vaccine, with the requested data expected to arrive with the agency this week. The FDA will then need additional time to analyse and consider the data, with no definitive timeline on how long this could take.