An expert from the World Health Organization has said a claim by former director of the U.S. Centers for Disease Control and Prevention (CDC) Robert Redfield that the novel coronavirus "escaped" from a lab in Wuhan was "damaging."

"I do not believe this somehow came from a bat to a human. And at that moment in time, the virus came to the human, became one of the most infectious viruses that we know in humanity for human to human transmission ... Normally, when a pathogen goes from a zoonotic to human, it takes a while for it to figure out how to become more and more efficient," Redfield told CNN in the documentary "COVID WAR: The Pandemic Doctors Speak Out," which aired on Sunday, but stressed he was "not implying any intentionality."

While still supporting the lab leak theory which the World Health Organization (WHO) said was "extremely unlikely," Redfield admitted there was no evidence. "That's my own feelings. And only opinion. I'm allowed to have opinions now."

His comments have sparked disagreement and criticism among scientists.

Peter Daszak, zoologist and member of the WHO-China research team, tweeted that such conspiracies are "damaging and without evidence."

Daszak said the WHO team examined lab work of a number of researchers in Wuhan and found "no evidence at all that any of the labs in China were working on this virus prior to the outbreak … Did they have viruses that were potential ancestor of SARS-CoV-2? Again, no evidence of that."

Dr Anthony Fauci, the director of the United States' National Institute of Allergy and Infectious Disease, said his former colleague was just expressing an opinion. He added that "there certainly are possibilities … of how a virus adapts itself to an efficient spread among humans."

"One of them [the possibilities] is in the lab," Fauci added. "And one of them – which is the more likely, which most public health officials agree with – is that it likely was below the radar screen, spreading in the community in China for several weeks, if not a month or more, which allowed it when it got recognized clinically to be pretty well-adapted."

The lab leak: a long, unsupported theory 

Though believers who repeatedly preached such claim with no substantial evidence that the pandemic is a result of the COVID-19 virus "escape" from the Wuhan Institute of Virology, experts and professionals, who have decades of virus research experience, firmly and repeatedly refuted such theory. 

Daszak shared his doubts about a "lab leak" in multiple interviews and occasions. 

In a well-known U.S. TV show "60 Minutes," Daszak said the virus was transmitted from a bat onto one of these wildlife farms, infecting the wild animals before they were shipped to the market. It was there that people who were handling such animals became infected.

He said several kinds of animals such as civets, ferret badgers, and rabbits are all local wild animals that can carry the virus. "Those animals were coming into the market from farms over 1,000 miles away." 

"Something like 75 percent of emerging diseases come from animals into people. We've seen it before. We've seen it in China with SARS," Daszak added. 

He said a lab leak means the virus needs to be in the lab, but WHO "never had any evidence of a virus like COVID in the lab." He admitted the WHO team did look into "lab leak" claims during the field trip but deemed it "extremely unlikely." 

Daszak also refuted that the team could've been lied to about the lab by the Chinese government. 

"You sit in a room with people who are scientists and you know what a scientific statement is and you know what a political statement is. We had no problem distinguishing between the two," he said. 

Besides Daszak, Peter Ben Embarek, the expert who led the WHO team in Wuhan, also ruled out the possibility of a "lab leak" during the press conference on the research findings in February. 

Their research in Wuhan concluded that the virus most likely jumped from an animal to an intermediary host and then humans. 

Read More: Animal origin of coronavirus not yet identified: China-WHO joint team

More experts are dismissing the leak theory. 

Jonna Mazet, an epidemiologist at the University of California, Davis, who has worked with and trained researchers from the Wuhan Institute of Virology in the past, said the spreading of the virus is "highly unlikely a lab accident," listing four reasons in an interview with Business Insider in May. 

Read More: U.S. researcher who worked in Wuhan virology lab says COVID-19 leak 'highly unlikely' 

Shi Zhengli, a virologist and researcher at the Wuhan Institute of Virology, told CGTN in May about the critical timeline of the virus spread.

She said the institute obtained samples of the virus on December 30, 2019 then they started studies and performed high-throughput sequencing of the samples and the isolation of pathogens.

"It showed that the pathogen's sequence was not the same as the already existing viruses. So we named it a novel coronavirus," Shi said. They submitted the genome sequence to the WHO on January 12, 2020, when China had began alerting the public about the virus. 

Read More: CGTN Exclusive: The story of COVID-19 in the words of Wuhan lab expert 

Wang Yanyi, an immunologist and director of the institute, called the "lab leak" theory a "pure fabrication." She said before December 30 2019, they had no knowledge about the virus, "or had we ever encountered, researched or kept the virus." 

"How could it have leaked from our lab when we never had it?" she said.  

In addition, Wang explained that although the institute has studied bat coronaviruses since 2004, the research has been focused on "source tracing of SARS," rather than the novel one.  

"We know that the entire genome of SARS-CoV-2 is only 80 percent similar to that of the SARS virus. It's an obvious difference." 

Read More: CGTN Exclusive: Director of Wuhan Institute of Virology says 'let science speak'