(CNN) AstraZeneca, whose Phase 3 coronavirus vaccine clinical trial has been on hold for more than a month, did not get critical safety data to the US Food and Drug Administration until last week, according to a source familiar with the trial.
The FDA is considering whether to allow AstraZeneca to restart its trial after a participant became ill. At issue is whether the illness was a fluke, or if it may have been related to the vaccine.
The source said the root of the delay is that the participant was in the United Kingdom, and the European Medicines Agency and the FDA store data differently.
"They had to convert data from one format to another format. It's like taking stuff off a PC and putting it onto an Apple. They had to spend a lot of hours to get what they wanted," the source said.
On Friday, a federal official hinted there might be some word this week on the trial's future.
"FDA continues to review safety information, and we'll just have to see over the next week or so where that turns out," Paul Mango, deputy chief of staff for policy at the US Department of Health and Human Services, said during a telebriefing with reporters.
The AstraZeneca trial started August 31, and the current trial pause was announced September 9. The trial has already resumed in the UK.
A former FDA commissioner told CNN he doesn't think computer issues are the reason it's taken AstraZeneca weeks to get the safety data to the FDA.
"It's definitely not a technology issue," said Dr. Robert Califf, who served as FDA commissioner from 2016 to 2017.
Califf said he thinks the FDA is being very careful and looking for more information, not just about the one participant whose illness spurred the current pause, but also the illness of another participant that caused a pause in July.
The University of Oxford, which is overseeing the UK arm of the trial, on its website describes study volunteers who "developed unexplained neurological symptoms including changed sensation or limb weakness."
"FDA may be asking for specialized tests or some kind of further follow up," Califf said. "And it's not just a matter of raw data. They want to hear expert opinions and aggregate all of this, and that can take some time."
There might be differences of opinion within AstraZeneca or within the FDA about what exactly happened to these participants and whether the vaccine is to blame, he added.
"There's a lot of judgment involved in these decisions," he said.
FDA Commissioner Dr. Stephen Hahn told Bloomberg News Wednesday that while he couldn't speak "to a specific application or discussion with a company," the FDA wants to do a thorough assessment of safety events.
"We have a give-and-take with companies. Sometimes that takes a short period of time. And sometimes that takes longer," Hahn said.
AstraZeneca did not respond when CNN asked when the safety information was sent to the FDA.
"AstraZeneca is continuing to work with the FDA to facilitate review of the information needed to make a decision regarding resumption of the US trial. It is up to the regulatory bodies to review and make approval decisions based on the data," according to a statement from a company spokesperson.
The source who said the information was handed over to the FDA last week, agreed with Califf that the agency is going to be very methodical about reviewing the data.
"I'm sure the FDA wants to make their decision as soon as they can, but they also need to make sure that decision is sound. It's critical to the American public. And it's critical that the American public has trust in the FDA. If we lose trust in the FDA, we're done," the source said.