A test person has suffered from a "possible unexplained illness", reports Reuters.
The tests of the vaccine are now paused and the company investigates whether the disease has been caused by the vaccine.
"A routine evaluation triggered a halt to vaccine testing to review safety," a spokeswoman for Astra Zeneca said in a statement, Reuters reports.
"Accelerate evaluation"
The company has not given a more detailed description of what kind of illness the test participant has suffered from, but the person is expected to recover.
"We are working to speed up the evaluation of the individual incident to minimize any impact on the testing schedule," Astra Zeneca wrote in the statement.
Sweden has concluded an agreement on the vaccine
The person affected by the disease is taking part in the tests carried out in the UK, together with researchers at Oxford University.
The vaccine in question is the vaccine that Sweden has previously agreed on.
The company's vaccine tests are also performed in the United States, Brazil and South Africa.
In the future, studies are also planned to be conducted in Russia and Japan.
The tests are now paused in all countries.
In total, around 50,000 people are planned to be tested in the study.
It is unclear when testing may resume.