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Johnson & Johnson's coronavirus vaccine trial has been temporarily put on hold after one of the participants developed an 'unexplained illness'.
This is the second time a vaccine trial, which is in the late stages of testing, has been affected by a major setback.
While the testing for J&J's potential Covid-19 vaccine will be paused, a top official says this is 'not at all unusual' when dealing with so many people.
The company has been trying to enrol 60,000 people to see whether its vaccine will prove effective against the pandemic.
No details have been released about the specific illness the participant suffered.
Johnson & Johnson said in a statement: "We must respect this participant's privacy. We're also learning more about this participant's illness, and it's important to have all the facts before we share additional information."
Mathai Mammen, the global head of Janssen research and development at J&J, added: "It's not at all unusual for unexpected illnesses that occur in large studies over their duration."
The patient's condition is being reviewed reviewed by an independent board as well as by company doctors. It's not yet known when the trial will be able to resume.
A UK company also trying to develop a vaccine suffered a similar fate last month when a participant had and 'adverse reaction' to the injection.
In a statement, AstraZeneca said it 'voluntarily paused' the trial to allow a review of the 'single event' by an independent committee.
"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," the statement said.
"In large trials illnesses will happen by chance but must be independently reviewed to check this carefully."
The trial, however, resumed within a week - and hopes remain high that the Oxford University/AstraZeneca vaccine might be one of the first on the market after its successful Phase 1 and 2 trials.
The vaccine is now in Phase 3 of trials, which has involved some 30,000 participants in the US as well as in the UK, Brazil and South Africa. Phase 3 trials in vaccines often involve thousands of participants and can last several years.
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