"Beginning of 2021, we may have a vaccine candidate that is actually safe and effective," Fauci told Zuckerberg during an hour-long Facebook live Thursday.
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So where are we right now?
"There are multiple candidates, not only one," he said. "But there will be more than one going into advanced trials in tandem sequentially."
According to Fauci, one of many promising options was published in the New England Journal of Medicine two days ago. The vaccine candidate is described as having the 'real bullets' or neutralizing antibodies in this case to fight the virus.
"That is the gold standard of protection is neutralizing antibodies," Fauci said. "It induced it at levels at a moderate dose of the vaccine that were as high as or higher than what you see with plasma after natural infection."
The closer the vaccine response is to natural infection - there's a greater chance the vaccine will be effective.
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While there will be many top contenders, Dr. Fauci said this particular vaccine is entering a Phase 3 clinical trial by the end of July. That trial that will have a testing pool of 30,000 people.
Assuming all goes well, there's potential a reliable vaccine could be available in the U.S. by the beginning of 2021. Before that, Dr. Fauci said it's possible to have an idea by late fall or early winter how many viable vaccine candidates will be available.
"I'm cautiously optimistic about this Mark," said Fauci.
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