coronavirus vaccine could be approved as early as Thanksgiving, Moncef Slaoui, the chief scientist leading the Trump administration's coronavirus vaccine initiative, told Fox News Tuesday. 

In a rare interview on "The Story", Slaoui said he expects two of the vaccines currently enrolled in Phase 3 clinical trials to reach a designated benchmark by "the end of this month and next month" before they can file for an emergency use authorization from the Food and Drug Administration.  

"Thanksgiving, shortly before or shortly after would be my best estimate," Slaoui told host Martha MacCallum, "but this is only, of course, a best estimate."

Slaoui, who has been the driving force behind Operation Warp Speed, a multiagency collaboration led by the Department of Health and Human Services, explained the unprecedented pace of development, reassuring skeptics who question the safety and efficiency of an expedited vaccine.

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"It's been exceptionally fast for a number of reasons," he said. "One is we have learned over the last 10, 15 years, a lot about what's called platform technologies, which is the basic of a number of vaccines whereby 80%, 90% of how you make a vaccine is already predefined. 

"So we use that platform technology that we knew could be fast, could be safe, could be manufactured at scale ... and that was one reason we went very fast."

More than 7.8 million have been infected by COVID-19 across the U.S., and at least 215,861 have died.

"The second reason is… we prepared, we took financial risk, we didn't take any safety risks and we were able to prepare for Phase 1 trial, Phase 2 trial, Phase 3 trial ... so we cut all of the blanks, and  less than 10 months after discovering this virus, we have six programs in the clinics, of which four are in phase three trials," Siaoui added.

Despite his optimistic projections, Slaoui he is "extremely worried about how politicized this situation is around the vaccine and how people are reacting to it. 

"We will be fully transparent, people will understand exactly the performance of the vaccine, their safety and the benefit," he said, "and I hope people will realize that the only way really to allow us to move away and control this pandemic would be through mass vaccination."

Slaoui's comments come after pharmaceutical giant Johnson & Johnson announced that it had paused its late-stage COVID-19 vaccine trial after an “unexplained illness” was reported in a participant, something he called "not unusual at all."

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"There are serious events in all clinical trials almost always," he explained. "Of course, they don't happen in the limelight with all the country looking into it. The key here is that every vaccine has its safety monitored very, very carefully, and trials are put on hold, [to] try to understand what's going on. If the vaccine is deemed to still be safe and can progress because the event was not really associated with vaccination, it will go on. If it is deemed to be a problem, the trials will be stopped."

Asked at one point whether someone can be infected with the coronavirus twice, Slaoui said, "exceptions exist always."

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"But," he added, "the exception isn’t the rule here. The rule is most people will be protected after being exposed or being vaccinated."