EXCLUSIVE: Robert Redfield, the former director of the Centers for Disease Control and Prevention, explained the reasoning behind his opinion that a lab leak was the most likely explanation for the COVID-19 pandemic’s origins, in a wide-ranging interview with Fox News.

The former CDC director discussed the ongoing debate over the COVID-19 pandemic’s origins during a lengthy interview this past weekend with Dr. Marc Siegel, a professor of medicine at the NYU Langone Medical Center and a Fox News contributor. While Dr. Anthony Fauci and other leading members of the science community have long argued that human contact with an infected animal started the pandemic, calls to investigate the lab leak theory have intensified in recent days.

Redfield argued COVID-19’s efficient human-to-human spread contradicted the behavior of other deadly coronaviruses with similar profiles, such as SARS and MERS, which first reached humans through animal contact but spread at a much slower pace.

"When I said before that I didn't think it was biologically plausible that COVID-19 went from a bat to some unknown animal into man and now had become one of the most infectious viruses," Redfield said, "That's not consistent with how other coronaviruses have come into the human species. And, it does suggest that there's an alternative hypothesis that it went from a bat virus, got into a laboratory, where in the laboratory, it was taught, educated, it evolved, so that it became a virus that could efficiently transmit human to human."

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Public calls to investigate the lab-leak hypothesis, once dismissed in China and by many in U.S. media as a conspiracy, have intensified since last month, when the Wall Street Journal reported three researchers at China’s Wuhan Institute of Virology sought hospital treatment for symptoms consistent with COVID-19 in late 2019. 

Redfield, a virologist, expressed disappointment in what he described as a "lack of openness" within the scientific community to "pursue both hypotheses."

"I'm just giving my best opinion as a virologist, and I don't think it's plausible that this virus went from a bat to an animal – we still don't know that animal – and then went into humans and immediately had learned how to be human-to-human transmissible to the point of now causing one of the greatest pandemics we've had in the history of the world," Redfield added.

President Biden said in May that the U.S. intelligence community had "coalesced around two likely scenarios" regarding the pandemic’s origins but had yet to reach a definitive conclusion. The president called on officials to present their best findings within 90 days.

Beijing has scrambled to deflect international scrutiny related to the lab leak theory, accusing U.S. officials of having political motivations in their calls for further investigation. 

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Redfield also expressed doubt about the integrity of the World Health Organization, which concluded in a joint report with China released in March that a lab leak was "extremely unlikely." He argued the WHO was "too compromised" by Beijing’s influence to conduct a truly transparent investigation.

"Clearly, they were incapable of compelling China to adhere to the treaty agreements that they have on global health, because they didn't do that. Clearly, they allowed China to define the group of scientists that could come and investigate," Redfield said. "That's not consistent with their role."

Redfield served as CDC director in the Trump administration from March 26, 2018, to Jan. 20, 2021.