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The chief executive of AstraZeneca says it is not uncommon for vaccine trials to be paused because of an adverse event and he believes the company can still deliver a safe COVID-19 vaccine by the end of the year.

“If you have an event that you didn’t expect, then you stop to look at it and explore it and study it,” Pascal Soriot said Thursday during a conference sponsored by British news outlet Tortois Media. “And that’s what happened in our case, but it’s really common.”

AstraZeneca announced this week that it had halted the Phase 3 trial of a COVID-19 vaccine the company is developing with researchers at Oxford University. It is considered one of the best candidates for a viable vaccine, but the trial was stopped after a female volunteer in Britain fell ill. She developed symptoms consistent with a rare disorder called transverse myelitis, which causes inflammation of the spinal cord. The woman’s condition has been improving, and she was expected to leave hospital this week.

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Mr. Soriot said an independent safety committee was reviewing the case and it had yet to determine if the volunteer had actually developed transverse myelitis. He added that the trial won’t restart until the review is completed. The committee can either allow the trial to continue or stop the project altogether.

This was the second time the trial for the vaccine, known as AZD1222, has been halted. Mr. Soriot confirmed that another pause happened in July when a participant experienced neurological symptoms. Further examination found the participant had multiple sclerosis, which was deemed unrelated to the vaccine.

Mr. Soriot said that although transverse myelitis is rare, it had surfaced in other vaccine projects. “What is true is that you will see transverse myelitis mentioned in the product information of several other vaccines,” he said. “And the issue with vaccines is you actually vaccinate thousands and thousands of people, so you will see all sorts of issues come up that are often not related to the vaccine.”

He added that many other vaccine trials have had to stop but such developments rarely get much public attention. “The difference with other vaccine trials is the whole world is not watching them, of course, so they stop, they study and they restart. What happened here is not uncommon,” he said. “What we have here is a very special set of circumstances where the entire world becomes involved in the conduct of the clinical trial and wants to know every step of the way.”

The Phase 3 trial of AZD1222 involves 30,000 participants in Britain, the United States, Brazil and South Africa. Such trials often last several years, but AstraZeneca has been able to speed up the process by working in parallel with regulators. Usually drug companies complete their trials and then submit the data to regulators, who spend months analyzing the information. AstraZeneca has been handing over data as the trials proceed.

“We could not do this for every single vaccine or drug because the system would not be supporting it – the regulator would not have the resources,” Mr. Soriot said. He added that the vaccine will ultimately be tested on as many as 60,000 people before it is approved.

Despite the setback, Mr. Soriot said that if the trial can restart soon the company is on track to deliver the vaccine in a few months. “We could still have a vaccine by the end of this year, maybe early next year, but by the end of this year is still possible.”

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Two other vaccines in development – by Pfizer Inc. and Moderna Therapeutics – are also in Phase 3 testing and could be ready by early 2021, he said.

AstraZeneca is producing the vaccine on a non-profit basis and has lined up manufacturing partners around the world to ensure it can be delivered in all regions at roughly the same time. The company and Oxford are also working with non-profit organizations to co-ordinate distribution among developing countries and to people in refugee camps.

Medical experts said it was not uncommon for trials to stop because a participant fell ill. “A suspension such as this is not unusual for Phase 3 trials, which have tens of thousands of participants,” said Ohid Yaqub, a senior lecturer at the science policy research unit at the University of Sussex.

“Suspending the trial gives time to investigate whether the incident is related to the vaccine or is happening by coincidence. If the data and safety monitoring board decides it’s the latter, the trial will resume. Other trials will also investigate their data, too.”

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