Brazilian regulator says testing of the vaccine will, however, continue even after the volunteer's death.
Brazilian health authority Anvisa has said that a volunteer in a clinical trial of the coronavirus vaccine developed by AstraZeneca and Oxford University has died, stating it had received data from an investigation into the matter.
The regulator said on Wednesday testing of the vaccine would continue after the volunteer's death. It provided no further details, citing medical confidentiality of those involved in trials.
The Federal University of Sao Paulo, which is helping coordinate phase 3 clinical trials in Brazil, separately said that the volunteer was Brazilian but did not say where the person lived.
AstraZeneca shares turned negative and were down 1.7 percent.
The federal government already has plans to purchase the UK vaccine and produce it at its biomedical research centre FioCruz in Rio de Janeiro, while a competing vaccine from China's Sinovac is being tested by Sao Paulo state's research centre Butantan Institute.
Brazil has the second deadliest outbreak of coronavirus, with more than 154,000 killed by Covid-19, following the United States. It is the third worst outbreak in terms of cases, with more than 5.2 million infected, after the United States and India.