A scientist whose statements have been extensively referenced in support of the lab leak theory of Covid-19's origin is now retracting his words.

Last month, Nobel laureate David Baltimore told the Bulletin of Atomic Scientists that a section of the coronavirus's genome - the furin cleavage site - was "smoking gun" evidence that it originated in a laboratory. The comment was rapidly picked up by a slew of believers in the hypothesis that the virus was created by humans.

Baltimore, though, backed down this week, telling the Los Angeles Times that he "should have softened the phrase 'smoking gun,' because I don't believe it proves the origin of the furin cleavage site, but it does sound that way."

"I believe that the question of whether the sequence was put in naturally or by molecular manipulation is very hard to determine, but I wouldn't rule out either origin," Baltimore said.

However, in recent months, the lab leak argument has gained popularity. Former director of the Centers for Disease Control and Prevention told CNN in March that the virus "most likely" originated at the Wuhan Institute of Virology, where scientists were studying coronaviruses.

"I am of the point of view that I still think the most likely ideology of this pathogen in Wuhan was from a laboratory," he said then. "Escaped. Other people don't believe that. That's fine. Science will eventually figure it out."

The Wall Street Journal reported in May three staff members at the Wuhan facility were hospitalized with Covid-like symptoms in November 2019 - earlier than China's first documented cases of the disease.

A few days after the Journal article was published, President Joe Biden directed intelligence authorities to look into the origins of Covid-19, including the likelihood that it originated in a Chinese laboratory.

Meanwhile, World Health Organization Director-General Dr. Tedros Adhanom Ghebreyesus has demanded a more detailed report on the pandemic's origins, insisting that all hypotheses "remain on the table."

Meanwhile, Tedros Adhanom Ghebreyesus, head of the World Health Organization, has demanded a more extensive report on the pandemic's beginning, and insists that all hypotheses "remain on the table."