If the illness was found to have been caused by the experimental vaccine, the investigators would then have to find out if it had to do with the adenovirus vector used in it, or the vaccine construct that was used to express the antigen, he said. The information would then be shared with other vaccine developers using similar technology, he added.

Farrar said there had been no similar incident in trials for other adenovirus vector vaccines.

“These adenovirus vectors have been in tens of thousands of people … I am not aware of any issue like this in any of the adenovirus vector trials that have been conducted over the last few years, in Mers, previously in Sars, or in Ebola.”

At the same briefing, Melanie Saville, director of vaccine research and development at the Coalition for Epidemic Preparedness Innovations, said in addition to the independent review, scientists would also look at the database of other vaccine candidates to check for any sign of a similar case in other trials. “It will take a little bit of time to sort out and find the path forward,” she said.

A statement issued by AstraZeneca on Tuesday said an independent committee was reviewing the safety data of the trials and that it was a “routine procedure”.

“We are working to expedite the review of the single event to minimise any potential impact on the trial timeline,” the statement said.

Scientists said putting a trial on hold was not uncommon. When and whether the trials could resume would depend on whether the independent committee and the Medicines and Healthcare Products Regulatory Agency in Britain could determine if the illness was caused by the inoculation.

“If it is related to the vaccine, then they may need more time to find the cause. They can resume as soon as they determine it is not an adverse effect of the vaccine. Of course, they have to negotiate with the regulator. They also have to determine how common such a case is in the trials,” according to Leo Poon Lit-man, head of the public health laboratory sciences division at the University of Hong Kong.

This is the second time the Oxford University/AstraZeneca coronavirus vaccine trials have been put on hold for independent investigation since they began in April.

According to an article posted by Oxford scientists on The Conversation website on Tuesday, the team and its partners have vaccinated about 17,000 people in Britain, Brazil and South Africa in trials, with half receiving another vaccine for comparison. Phase 3 trials also started in the US in August and the company is planning to enrol up to 50,000 volunteers worldwide for the last stage of trials.

All of AstraZeneca’s international vaccination trials – in Britain, Brazil, South Africa and the US – are now on hold.

A report by The New York Times said a volunteer in Britain had suffered from transverse myelitis, a spinal inflammation, but that has not been confirmed by the company.

Medical website STAT News, which broke the news, reported on Thursday that AstraZeneca’s chief executive, Pascal Soriot, told a private conference call with investors on Wednesday that the patient has experienced neurological symptoms consistent with transverse myelitis, but that the diagnosis was not confirmed yet. He also said the woman was recovering.

AstraZeneca and Oxford University did not give further information about the nature of the illness, or provide more information about the patient, when contacted by the South China Morning Post.

If it is found to be transverse myelitis, that could be a serious illness, said Helen Petousis-Harris, associate professor of vaccinology at the University of Auckland, but she noted the condition could be caused by other factors.

“While not yet confirmed, it is biologically possible to be related to the vaccine. It is also more likely to be caused by something else, particularly a viral infection,” she said.

“There are many infectious agents that can trigger this kind of event, such as influenza and the herpes viruses. Vaccines are a plausible trigger but less likely.”

Given that thousands of volunteers take part in large-scale clinical trials, it is possible that some might have unknown health problems.

“When you have what will ultimately be hundreds of thousands of people under observation in clinical trials, some will get sick, especially when you start recruiting older people into these studies,” Petousis-Harris said. “When this happens sometimes the trial will need to be halted while the event is investigated, and a lot of questions answered.”

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She said trials of other potential Covid-19 vaccines were unlikely to be affected at this stage, especially those using other technologies.

“It does not have bearing on trials of other types of vaccine, such as the RNA vaccines,” she said.

According to Antonio Bertoletti, a professor of emerging infectious diseases at the Duke-NUS Medical School in Singapore, the halt showed that the developer was being careful. “This stop demonstrates that the group developing this vaccine is doing its work very seriously,” he said.

On Tuesday, top executives of nine pharmaceutical companies signed a joint pledge that any coronavirus vaccine they produced would be developed and tested with “high ethical standards and sound scientific principles”.

But China revealed on Monday that it had already inoculated hundreds of thousands of people under “emergency use authorisation” granted to three vaccine candidates after they completed phases 1 and 2 of trials, and said there had been no adverse event so far.

Petousis-Harris said the hold on the Oxford/AstraZeneca trial was a reminder of the importance of safety and she cautioned against emergency use before trials are completed.

“I think the halting shows that the study is not being rushed. It is a reminder that safety is being taken very seriously,” she said. “I do not think these vaccines should be deployed before completing their clinical testing. Once they are given to the general population there must be robust safety monitoring systems in place so that monitoring can continue.”