AstraZeneca (AZ) has released the trial protocol for the late-stage study of its coronavirus vaccine candidate AZD1222 as its investigation into an adverse event continues.

According to the protocol, the phase 3 trial has set a bar of 50 percent efficacy for the vaccine candidate, the same percentage the FDA has said it expects COVID-19 vaccines to meet. The protocol also mandates an interim efficacy analysis after 75 participants have become infected with coronavirus.

“A statistically significant finding at the interim analysis will not be considered a reason to stop the study, but instead will be interpreted as early assessment of efficacy,” the protocol says.

It is not known if AZ plans to file for regulatory approval or Emergency Use Authorization if the trial’s interim efficacy analysis shows the vaccine meets the effectiveness threshold. The company did not respond to a request for comment.

The drugmaker, the third to publish the workings of its vaccine trial after Moderna and Pfizer, came under scrutiny after a UK participant in its phase 3 vaccine trial experienced a serious adverse event (CenterWatch Weekly, Sept. 14).

The National Institutes of Health has launched its own investigation into the adverse event and urged the UK drugmaker to be more open about the potential side effects.