Pharmaceutical major AstraZeneca, who is developing a coronavirus vaccine in association with Oxford university, has paused their trials after a participant fell ill due to a "suspected adverse reaction", the company stated, as reported by medical journalStat News. "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. 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 sometimes happen by chance but must be reviewed independently. We are working to expedite the review of the single event to minimise any potential impact on the trial timeline," according to the statement.

The Oxford-AstraZeneca vaccine is the most advanced one in the world, well into the third and final phase of human trials. The Moderna vaccine candidate is a close second.AstraZeneca's US-traded shares fell more than six per cent in after-hours trading following reports of the trial being paused.

Vaccine testing is a four-stage processpre-clinical testing on animals; Phase I clinical testing on a small group of people to determine its safety and to learn more about the immune response it provokes; Phase II trials, or expanded safety trials, where dosage and frequency will be tested across wider cross-sections of the population; Phase III large-scale tests where the vaccine is administered to thousands of people to confirm its efficacy. Phase I and Phase II are the early trials, which will then be followed by a rigorous, intensive Phase III clinical testing, where the longevity of the vaccine response (whether the vaccine will last for long periods of time) will be analysed.

What does an 'adverse event' mean?

According to United States Food and Drug Safety Administration (FDA), 'adverse events' are not necessarily side effects caused by vaccination. An adverse event is a "health problem that happens after vaccination that may or may not be caused by a vaccine". These events may require further investigation.

By definition, a side effect has been shown to be linked to a vaccine by scientific studies. While most vaccine trials induce some form ofpain at the injection site, hyperthermia, headache, asthenia (weakness or lack of energy), and muscle and joint pain in its participants, an adverse event would equate to something slightly more serious, possibly requiring hospitalisation.

On the flip side, temporary holds of large medical studies aren't unusual, and investigating any serious or unexpected reaction is a mandatory part of safety testing. AstraZeneca pointed out that it's possible the problem could be a coincidence; illnesses of all sorts could arise in studies of thousands of people.

During the third and final stage of testing, researchers look for any signs of possible side effects that may have gone undetected in earlier patient research. Because of their large size, the studies are considered the most important study phase for picking up less common side effects and establishing safety.

Dr Ashish Jha of Brown University said via Twitter that the significance of the interruption was unclear but that he was "still optimistic" that an effective vaccine will be found in the coming months. "But optimism isn't evidence," he wrote. "Let's let science drive this process."

Angela Rasmussen, a virologist at Columbia University in New York, tweeted that the illness may be unrelated to the vaccine, "but the important part is that this is why we do trials before rolling out a vaccine to the general public".

How does the vaccine work?

The vaccine candidateAZD1222, according to AstraZeneca, uses a "replication-deficient chimpanzee viral vector based on a weakened version of a common cold (adenovirus) virus" that causes infections in chimpanzees and contains the genetic material of SARS-CoV-2 spike protein. After vaccination, the surface spike protein is produced, priming the immune system to attack COVID-19 if it later infects the body.

AstraZeneca has struck a deal with Europe's Inclusive Vaccines Alliance to supply up to 400 million doses of an experimental COVID-19 vaccine. The agreement struck Saturday aims to make the vaccine available to other European countries that wish to take part.

Late last month, AstraZeneca began recruiting 30,000 people in the US for its largest study of the vaccine. It also is testing the vaccine, developed by Oxford University, in thousands of people in Britain, and in smaller studies in Brazil and South Africa.

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. Those two vaccines work differently than AstraZeneca's, and the studies already have recruited about two-thirds of the needed volunteers.

The "adverse reaction" came the same day that AstraZeneca and eight other drugmakers issued an unusual pledge, vowing to uphold the highest ethical and scientific standards in developing their vaccines. The announcement follows worries that President Donald Trump will pressure the US Food and Drug Administration to approve a vaccine before it's proven to be safe and effective.

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Oxford COVID-19 vaccine trial paused after 'unexplained' illness in participant. What does it mean? - THE WEEK