More than 17 months after the emergence of the worst pandemic in more than a century, scientists are yet to figure out the origins of the virus. The first cases of Covid-19 were reported in Wuhan, China, last year, and claimed more than 35 lakhs deaths worldwide.

The investigation into the origins of the virus has once again gained momentum after US President Joe Biden, on Wednesday, ordered U.S Intelligence agencies to redouble their efforts to investigate the origins of Coronavirus and report back to him in 90 days.

“I have now asked the intelligence community to redouble their efforts to collect and analyze information that could bring us closer to a definitive conclusion,” President Biden said. “The United States will also keep working with like-minded partners around the world to press China to participate in a full, transparent, evidence-based international investigation and to provide access to all relevant data and evidence,” he added.

The development comes amid a growing controversy over a lab-leak theory according to which the virus was leaked from a bio laboratory in China. There are growing calls for the World Health Organization to carry out the second phase of its investigation into the origins of the Coronavirus with independent experts given full access to original data and samples in China.

Earlier this year, a WHO-led team had spent four weeks in Wuhan with Chinese researchers to understand the origin of the virus. It said in a report that the virus had probably been transmitted from bats to humans through another animal, and that “introduction through a laboratory incident was considered to be an extremely unlikely pathway.” However, the US has claimed that the initial WHO study was “insufficient and inconclusive.”

Meanwhile, China's state media have accused the US government and Western media of spreading rumours. China has hit back at suggestions that the virus may have escaped from a laboratory and said that the United States was playing politics over the issue. The Chinese foreign ministry pointed that scientists from the WHO had already said the ‘lab-leak’ was highly unlikely.

“Some people in the United States completely ignore facts and science, foreign ministry spokesperson Zhao Lijian said on Thursday.

The scientific and political debate about how the virus became a global threat to humans has been simmering since the beginning of the pandemic. Although deep uncertainty still remains about the origins of a virus, two prevailing competing theories are doing the rounds for almost a year now.

The lab-leak theory

For some scientists, the leak of Coronavirus, accidentally or otherwise, from a Wuhan Lab, China’s leading SARS research facility, is a plausible hypothesis in regards to the origin of the virus. Most argue that the Wuhan Institute of Virology (WIV), which has been studying coronaviruses in bats for over a decade, is in very close proximity with the Huanan Seafood Market, which reported the first cluster of infections in Wuhan, China, the centre of the outbreak.

Scientists who support this theory argue that the virus could have leaked from WIV and spread to the Huanan Seafood market. Several scientists have also lately begun speaking out about the need to remain open to the possibility that the virus had accidentally emerged from a lab.

There is also growing consensus among experts that this theory, which was earlier described as a conspiracy theory, should be looked at more closely, especially with the Chinese government’s refusal to allow the lab-leak scenario to be fully investigated.

The Wall Street Journal had reported on Sunday that an undisclosed U.S. intelligence report has stated that three researchers from WIV became sick in autumn 2019, before the first identified case of the outbreak, with symptoms consistent with both Covid-19.

The animal-to-human transmission theory

On the other hand, many scientists have favoured the natural origin theory and said that it is more plausible than a lab leak. According to this theory, the virus originated in bats, jumped to another animal, and then mutated in a way that enabled it to transmit to humans, and from human to human.

This hypothesis was widely accepted by the experts at the beginning of the pandemic but many have started to question it because scientists have not found a virus in either bats or another animal that matches the genetic make-up of Covid-19.

What did the WHO team found in Wuhan?

Earlier in January this year, a WHO-led team of experts of 17 scientists from around 10 countries along with 17 Chinese scientists looked for clues for almost 27 days in and around Wuhan to understand the origin of the Coronavirus pandemic. The team visited the Huanan seafood market, government laboratories and interviewed Chinese officials. However, their finding did not produce any conclusive evidence and asserted the hypothesis from the initial days of the pandemic.

The 124-page report of a joint inquiry by the World Health Organization and China dismissed the possibility of the virus emerging accidentally from a Chinese laboratory and stated that Coronavirus appeared in humans after jumping from an animal.

However, the WHO report was heavily criticised by western power over doubts regarding the methodology of the report and China involvement in it. WHO Director-General Tedros Adhanom Ghebreyesus said “I do not believe that this assessment was extensive enough” and that the lab-leak theory needed more investigation.