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A Phase 3 vaccine developed by drug manufacturer AstraZeneca was put on hold when a volunteer in the UK fell ill after suffering a "serious adverse reaction" to the vaccine.

The vaccine being developed by drug manufacturer AstraZeneca and the University of Oxford in England was put on hold after a UK participant experienced a "serious adverse reaction".

The adverse reaction was described only as an "unexplained illness" in one of the trials.

A spokesperson for AstraZeneca, the frontrunner in the race for the first Covid-19 RNA vaccine, said in a statement on Tuesday that the company's "standard review process triggered a pause" to allow for a "review of safety data."

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Another source within the company, who spoke to Statnews.com on condition of anonymity, said the adverse reaction is having an impact on other AstraZeneca vaccine trials, as well as clinical trials being conducted by different vaccine manufacturers.

Nine drug manufacturers have vaccine studies currently under way in the U.S. and abroad.

The U.S. is currently testing vaccines at 62 sites across the country, according to clinicaltrials.gov.

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AstraZeneca's clinical trial is the first to reach the critical Phase 3 stage - the final step before the vaccine gets FDA approval. The normal timeframe for clinical trials is three years.

Adverse reactions to vaccines that require hospitalization may include life-threatening illness and even death.

Doctors in the U.S. are concerned that the vaccines are being rushed to market without adequate time to determine side effects.

Although healthcare professionals are due to get the vaccines first, many doctors and nurses say they will decline to take a vaccination that is rushed to market -- particularly since the Covid death rate is way below 1%.

Fully one third of Americans say they will refuse the vaccine if one is available before the election in November.

News of the adverse reaction sent AstraZeneca's stock tumbling 8% in after hours trading on Tuesday.