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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.

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Kamala Harris refuses to say whether she would take the RNA vaccine if it becomes available in October or November.

President Trump has insisted a vaccine will be available by the end of the year, and potentially before the November 3 elections.

"It will be delivered before the end of the year, in my opinion, before the end of the year, but it really might even be delivered before the end of October," Trump said Thursday. "How do you like that? Wouldn't that be nice?"

But the Democratic vice presidential candidate said she doesn't trust Trump's word on the safety or efficacy of a vaccine that was rushed through three years' worth of clinical trials in a matter of months.

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In an interview with CNN, Harris said, "I would not trust Donald Trump and it would have to be a credible source of information that talks about the efficacy and the reliability of whatever he's talking about. I will not take his word for it."

Alarm bells went off when the Centers of Disease Control and Prevention (CDC) sent letters to governors in every state requesting that states consider "waiving requirements" to allow vaccine distribution sites to be fully operational by November 1st.

Unlike normal vaccines, RNA vaccines are "encoded" with genetic "instructions" that tell your cells what to do.

Many Americans expressed concern that the CDC might be subjecting them to a potentially harmful vaccine.

Dr. Anthony Fauci, who sits on the White House Coronavirus Task Force, doesn't share their concern.

In an interview with Kaiser Health News earlier this week, Fauci said that Phase Three of the clinical trials could end earlier than expected, if the results prove "overwhelmingly positive."

Fauci told the publication that the Data and Safety Monitoring Board had a "moral obligation" to end the third phase of clinical trials early if the results were significant.

"I'm not concerned about political pressure," he added.

Fully one third of Americans say they would refuse the RNA vaccination even if it was deemed safe by the FDA and distributed to all 50 states before the election.

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A Black Atlanta woman signed up for Coronavirus vaccine clinical trials to ensure Black people are represented.

Ashley Nealy, 32, applied to participate in late-stage Covid-19 vaccine clinical trials by answering a social media ad looking for diverse volunteers.

Nealy tells Channel 2 Action News anchor Jovita Moore that she felt it was important to make sure the clinical trials were as diverse as possible.

"I actually wanted to do a clinical trial before I even saw the ad. I had registered at the website that Dr. Fauci mentioned, the Covid prevention network, and I never heard a call back. So whenever I saw the ad, I was like, let me just see if they'll accept me for this trial. So I signed up and less than two hours later actually got a call to participate in the trial," Nealy said.

Nealy said she was aware that there aren't enough Black volunteers for the trials and that there cannot be a vaccine product without more Black people. "So I figured that I can put my name in a hat and see if I can help be part of that," she said.

The vaccines contain RNA (Ribonucleic acid) that use specific instructions to tell human cells to produce antibodies against the coronavirus.

The RNA vaccine differs from normal vaccines that contain dead virus particles that trigger the immune system to produce antibodies against a specific pathogen.

RNA vaccines have never been approved for human use before because of the potential for something to go wrong.

What if human cells misread the mRNA instructions and go haywire inside the body?

Nealy says she participated in the trials knowing the RNA vaccine could negatively impact her health.

"This is so that I can help stop the pandemic and make sure that the vaccine works for Black Americans and everyone whenever it comes out," she told Moore.

"You know, so many people will say that they sort of have a mistrust," said Moore. "You're like a guinea pig. You don't know what's going to happen. You don't know what you're being injected with. What would you say to those people who have a real fear or maybe just a lack of interest right now?"

"Yeah, I will say I definitely understand," Nealy responded. "I know Black people in particular have a really long mistrust history with public health and with us being experimented on. And I understand that 100%. I will say if you are willing, and maybe if you're like a guinea pig like me to definitely participate, because we really can't move forward on this pandemic without knowing that a vaccine works for all of us."

Nealy says so far there have been no side effects.

"So actually, the next day after getting the vaccine, I did feel tired. I wasn't expecting to feel that fatigue. And I did have some body aches and sweating. And that was some of the things that they said you might experience if you have the vaccine."

Some volunteers received placebos while others were given the real vaccine.

"And then of course, they can't tell us but I'm pretty sure I did. And those are the only symptoms I had. They only lasted about a day and the next day I was fine," said Nealy.

The Centers for Disease Control (CDC) has asked all 50 states to prepare for massive distributions of Covid-19 vaccines by late October or early November.

Further complicating the vaccination effort is the cold storage requirement and the fact that two vaccine doses will need to be given 2 weeks apart.

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The Centers for Disease Control (CDC) has asked states to prepare for massive distributions of Covid-19 vaccines by early November.

Public health officials want the most vulnerable high-risk groups to get the Covid-19 vaccine as soon as late October or early November.

Officials agree that health agencies in all 50 states should "urgently" prepare for a complex effort to distribute the vaccines to "hundreds of millions" of Americans - despite the fact that the death rate is very low.

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The CDC sent guidance to every state on the same day President Trump told the Republican National Convention that a vaccine might be ready before the end of the year.

Over a dozen companies have accelerated clinical trials in a race to get their vaccines to market first. The usual safeguards have been waived in order to get the vaccines to market in a matter of months.

The FDA normally requires three years of clinical trials before a vaccine goes to market.

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The vaccine contains RNA (Ribonucleic acid) that are designed to alter the body's host cells to produce antibodies against the coronavirus.

Unlike normal vaccines, which contain DNA fragments of dead virus to produce antibodies, RNA vaccines are genetically engineered messenger RNA that contain specific directions to alter human DNA and tell it to create proteins, which in turn stimulates the cells to make antibodies.

RNA vaccines don't require dead pathogens to make antibodies. The human host's own genetic code is theoretically supposed to be altered (changed) by the RNA vaccine. RNA vaccines have been used on animals in veterinary medicine for years.

No RNA vaccines have ever been approved for human use.

Dr. Anthony Fauci and and Dr. Stephen Hahn, who heads the Food and Drug Administration, have said in interviews that RNA vaccines should be made readily available for certain groups, i.e. Black people, the elderly over 65, and "those incarcerated", before clinical trials have been completed.

Doctors and nurses on the frontlines will get the vaccines first, according to Fauci.

With so few Black people willing to step up and volunteer to be guinea pigs, the CDC's guidance acknowledged that its distribution plan is "hypothetical".

Dr. Saskia Popescu, an infection prevention epidemiologist based in Arizona, is concerned that the vaccine is highly politicized.

"It's hard not to see this as a push for a pre-election vaccine," he said.

Further complicating the vaccination effort is the cold storage requirement and the fact that two doses will need to be given 2 weeks apart.

"How are you going to make sure people get both [doses]?" said Dr. Cedric Dark, an emergency medicine physician at Baylor College of Medicine in Texas.