Trials of a Covid-19 vaccine under development by AstraZeneca and Oxford University have been put on hold owing to a reported side effect in a patient in the UK.
AstraZeneca issued a statement last night saying the late-stage studies of the vaccine had been paused while the company investigates whether the patient's reported side effect is connected with the vaccine.
AstraZeneca did not reveal any information about the patient's condition other than to describe it as "a potentially unexplained illness".
News site Stat first reported the pause in testing and said the possible side effect occurred in a testing volunteer in Britain, who was expected to recover.
The vaccine, developed by Oxford University, is being tested in thousands of people in Britain and the US, and in smaller study groups in Brazil and South America.
An AstraZeneca spokeswoman said the pause was part of a standard review process that occurs in trial if there is a "potentially unexplained illness" reported in any trial subject, and that the subject's illness could also be coincidental.
"As part of the ongoing randomised, controlled global trials of the Oxford coronavirus vaccine, our standard review process was triggered and we voluntarily paused vaccination to allow review of safety data by an independent committee", the spokeswoman said in a statement.
"This is a routine action which has to happen whenever there is a potentially unexplained illness in one of the trials, while it is investigated, ensuring we maintain the integrity of the trials.
"In large trials illnesses will happen by chance but must be independently reviewed to check this carefully.
"We are working to expedite the review of the single event to minimise any potential impact on the trial timeline. We are committed to the safety of our participants and the highest standards of conduct in our trials."
No details about the patient suffering the potential side effect, or the nature of the reaction, were given.
Temporary holds of large medical studies are not uncommon, and looking into any unexpected reactions is a mandatory part of safety testing.
Two other vaccines are in huge, final-stage tests in the United States, one made by Moderna Inc and the other by Pfizer and Germany's BioNTech.