Drugmaker AstraZeneca’s Covid-19 vaccine trial in the United States is still on hold after a participant developed a serious illness, but the Food and Drug Administration commissioner won’t say why.

At the US Pharma and Biotech Summit hosted by the Financial Times Wednesday, US FDA Commissioner Dr. Stephen Hahn was asked why the trial was still on pause in the US, even though it has resumed in Europe. Hahn said he couldn’t answer.

“I can’t speak to confidential commercial information and this summit knows that all too well,” he said.
“Our agency career officials in general take the issue of serious safety very seriously, the protection of the American people, very seriously ... We will look at any and all data, and we will make a decision when we have the data available to us regarding any issue, whether it's safety or effectiveness."

Hahn said he could not talk about a “specific issue” that may involve the AstraZeneca trial. 

“What I can tell you is that FDA is on the job with respect to looking at all safety and efficacy issues on all medical products,” he said.

An AstraZeneca spokesperson told CNN earlier that regulators in Britain, Brazil and South Africa had all decided to resume the trials. “We are continuing to work with the FDA to facilitate review of the information needed to make a decision regarding resumption of the US trial,” the spokesperson said.

“Regulators in each individual country determine when trials can start and they do this in their own time frame.”