Brazilian health authority Anvisa said on Wednesday that a volunteer in a clinical trial of the COVID-19 vaccine developed by AstraZeneca and Oxford University had died but added that the trial would continue.

Oxford confirmed the plan to keep testing, saying in a statement that after careful assessment "there have been no concerns about safety of the clinical trial." Brazilian newspaper O Globo reported that the volunteer had been given a placebo and not the trial vaccine, citing unnamed sources.

Anvisa provided no further details, citing medical confidentiality of those involved in trials.

AstraZeneca declined immediate comment.

The Federal University of Sao Paulo, which is helping coordinate phase 3 clinical trials in Brazil, separately said the volunteer was Brazilian without revealing where the person lived.

AstraZeneca’s American depositary receipts (AZN) fell as much as 3.3% in New York on Wednesday afternoon after word of the person’s death, but trimmed their losses in recent trading.

The federal government has plans to purchase the UK vaccine and produce it at its biomedical research center FioCruz in Rio de Janeiro, while a competing vaccine from China's Sinovac Biotech Ltd is being tested by Sao Paulo state's research center Butantan Institute.

Brazil has the second deadliest outbreak of coronavirus, with more than 154,000 killed by COVID-19, following the United States. It has the third largest number of cases, with more than 5.2 million infected, after the United States and India.