AstraZeneca COVID-19 coronavirus vaccine

In this photo illustration a medical syringe is seen with AstraZeneca company logo displayed on a screen in the background.

(CNN) -- A volunteer in Brazil's trial of AstraZeneca's experimental coronavirus vaccine has died, the Brazilian health agency Anvisa announced on Wednesday, but organizers said there was no reason to stop the trial -- an indication that the death is not linked to the vaccine.

Anvisa said that it was notified of the death on Monday but the International Evaluation and Security Committee overseeing the trial recommended that the trial continue. It was not clear whether the volunteer received the vaccine or a placebo shot as part of the trial, and Anvisa said no more information was being released for reasons of medical privacy.

"All significant medical incidents, whether participants are in the control group or the Covid-19 vaccine group, are independently reviewed. Following careful assessment of this case in Brazil, there have been no concerns about safety of the clinical trial and the independent review in addition to the Brazilian regulator have recommended that the trial should continue," Oxford University told CNN in an emailed statement on Wednesday.

So far, the D'Or Institute, which is administering the clinical trial in Rio, said 8,000 volunteers had received either the vaccine or a placebo in the trial.

"The rigorous analysis of data collected up to now did not raise any doubts about the safety of the study, and thus recommended that it continue," the Institute said in a statement.

A spokesman for vaccine maker AstraZeneca declined to comment specifically on reports that a volunteer in its trial of a coronavirus vaccine in Brazil had died, but indicated nothing had happened to justify stopping or pausing the trial.

"We cannot comment on individual cases in an ongoing trial of the Oxford vaccine as we adhere strictly to medical confidentiality and clinical trial regulations, but we can confirm that all required review processes have been followed," the spokesman told CNN.

"All significant medical events are carefully assessed by trial investigators, an independent safety monitoring committee and the regulatory authorities. These assessments have not led to any concerns about continuation of the ongoing study."

Vaccine experts note that volunteers in clinical trials can become ill or die for any number of reasons, and they may not be related to the vaccine.

"Without details it's impossible to know what has happened in this case but as the trial is continuing, I think we can assume the circumstances of the death were such that it was clearly not vaccine related," Ian Jones, a professor of virology at Britain's University of Reading, said in a statement.

"What we have to remember is that in any large trial the normal processes of morbidity (sickness) and mortality are still operating and that sometimes an event will occur in a trial participant which would have occurred anyway, trial or not." 

AstraZeneca vaccine trial paused previously

A death in a trial requires investigation to confirm whether the volunteer received the vaccine or a placebo -- and whether the death was due to trial participation or unrelated reasons.

AstraZeneca's vaccine continues testing in tens of thousands of volunteers around the world, but the recent news of a death is now the third adverse announcement from the trial.

In July, AstraZeneca put its trial on a "brief pause" while a safety review took place to investigate a volunteer's illness. That volunteer's condition wasn't announced until later, when AstraZeneca wrote in an email to CNN that the patient was found to have "an undiagnosed case of multiple sclerosis" and an independent panel concluded the condition was "unrelated to the vaccine."

Then in September, AstraZeneca announced it had paused global trials of its coronavirus vaccine because of an illness in another volunteer. AstraZeneca's Phase 3 US trial started August 31, and then the trial pause was announced September 8.

In mid-September, CNN obtained an internal safety report by AstraZeneca that shed light on the condition suffered by the volunteer.

The report detailed how the volunteer, a previously health 37-year-old woman, "experienced confirmed transverse myelitis" -- an inflammation of the spinal cord -- after receiving her second dose of the vaccine and was hospitalized on September 5.

The trial has resumed in the UK but remains on pause in the US.

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