The trial has been on hold in the United States for more than two weeks, and regulators face the task of trying to figure out if these illnesses were random and disconnected from the vaccine, or if they are related to the vaccine.
AstraZeneca and its partner, the University of Oxford, have given various accounts of the sick volunteers.
AstraZeneca told the media that the first volunteer who became ill had "an undiagnosed case of multiple sclerosis," but information on the Oxford website describes volunteers who developed "unexplained neurological symptoms." The company has stated that the second volunteer had "an unexplained illness" but an internal company document, which was labeled as "initial," states that at least at one point, she had a rare neurological condition called transverse myelitis.
"Those are very different statements," said Dr. Harlan Krumholz, a professor at the Yale School of Medicine, after reading accounts from AstraZeneca and Oxford about the first participant's condition. "Their information should be consistent, and when it's not consistent, that raises questions."
Krumholz and other scientists said the nature of the participants' neurological illnesses matter. If the two study subjects had similar conditions, that would raise more questions about whether the vaccine was to blame. If their conditions are different, that might suggest that they were random and not the result of the vaccine.
Mark Slifka, an immunologist, said it's particularly concerning because information posted on Oxford's website said both participants suffered neurological illnesses.
"That's why this is so shocking to me -- these are both neurological diseases, and that by itself is a red flag," said Slifka, a professor at the Oregon Health & Science University.
On its website, Oxford wrote that the volunteers' illnesses "were either considered unlikely to be associated with the vaccine or there was insufficient evidence to say for certain that the illnesses were or were not related to the vaccine."
AstraZeneca's CEO, Pascal Soriot, said at an event Thursday that "we should place safety at the center of everything you do."
Participants' illness related or random?
The clinical trial for AstraZeneca's coronavirus vaccine has already resumed in other countries, including the UK. A company spokesperson said they're working with US regulators to resume the trials in the US as well.
"We are continuing to work with the FDA to facilitate review of the information and the agency will decide when the US trial can resume," the spokesperson wrote in an email.
An FDA spokesperson declined to comment, referring CNN to AstraZeneca.
The nation's top infectious disease doctor told CNN on Monday that AstraZeneca and Oxford will have to carefully watch for any other volunteers who develop similar symptoms.
"What this triggers is extra alertness as trial goes forward -- if it goes forward -- on the part of the clinical trials sites to look out for anything even remotely similar, to see if there are any additional cases," said Dr. Anth
