The World Health Organisation (WHO) team investigating the origins of the coronavirus has wrapped up its 28-day mission to Wuhan, China, without finding the origin of the virus.

Experts believe the disease, which has recorded over 109 million cases and killed 2.4 million people worldwide, most likely infected humans from an intermediate animal host. After the first clusters of Covid-19 were detected in Wuhan in late 2019, the government put the city of 11 million people under a 76-day lockdown with strict restrictions on movement.

The WHO visit has not been short of controversies, with the probe plagued by delays, concerns over access and bickering between Beijing and Washington. Here are answers to some frequently asked questions about the issue.

WHO ARE THE EXPERTS?

Led by Dr Peter Ben Embarek, the WHO scientist for food safety and zoonosis, the 14-member investigative team consists of epidemiologists, animal and human disease experts, veterinarians, medical doctors and virologists.

They include zoologist Peter Daszak; infectious disease expert Dominic Dwyer; Dr Marion Koopmans, a virologist from the Erasmus University Medical Centre in the Netherlands; and Danish epidemiologist Thea Kolsen Fischer.

WHAT IS THE TEAM'S MISSION?

Apart from the laboratory leak hypothesis, the other three scenarios the WHO-led team explored with their Chinese counterparts were that the virus had jumped directly from an animal host; that it infected humans via an intermediate animal species; and that its transmission to the human population was via frozen food products.

In the course of its investigation, the team visited key sites like the Huanan seafood market, where many of the first cases were discovered, as well as the Wuhan Institute of Virology, which is involved in coronavirus research.

DID CHINA UNDERMINE THE W.H.O. PROBE?

Western nations have criticised Beijing for not being fully transparent at the start of the outbreak, mishandling its initial stages, and allowing the pathogen to escape beyond China's borders to ravage the world. It did not help that the experts' visit to Wuhan was delayed by China, leading to a rare rebuke from WHO director-general Tedros Adhanom Ghebreyesus early last month that he was "very disappointed" Beijing has not given the green light.

There were also worries about the team's access to data a year after the outbreak, amid concerns that the Chinese will try to stop embarrassing information from being released. Although several WHO team members insisted they were granted full access to the sites and people they requested to visit, some said they were not given raw data and instead had to rely on analyses by Chinese scientists.

WHAT IS THE CONTROVERSY OVER RAW DATA ACCESS?

Dr Dwyer said on Feb 13 that China's scientists refused to share raw data of the outbreak. The WHO team had requested raw patient data on 174 cases from Wuhan but were provided with only a summary.

In an interview with Agence France-Presse, Dr Embarek said the team would have been keen to have access to raw data about earlier cases of illnesses, including pneumonia, flu and fever, that could conceivably have been Covid-19.

However, team member Dr Daszak tweeted that he had a different experience. "I found trust & openness w/ my China counterparts. We DID get access to critical new data throughout. We DID increase our understanding of likely spillover pathways," he wrote.

To date, the Chinese government has not addressed the data issue.

HOW TRUE IS THE LAB LEAK THEORY?

The WHO team said it is extremely unlikely that the virus leaked from the Wuhan Institute of Virology and further research into this theory was not required.

The WHO statement will vindicate Chinese officials, who have long bristled at the suggestion that the virus could have come from a lab. Former US president Donald Trump had indicated, without evidence, that the virus could have been leaked from a Chinese lab, and has accused China of spreading Covid-19 around the world.

WHAT ABOUT THE FROZEN FOOD THEORY?

The Chinese authorities have been pushing the narrative that the virus could have been brought into Wuhan via cold-chain logistics and frozen food products, linking the theory to various domestic outbreaks in the past few months.

WHO mission members appeared to give weight to China's theory that the virus could be carried on cold-chain products, although foreign experts, including top WHO emergency expert Mike Ryan, and the US Centres for Disease Control and Prevention have downplayed the risk.

Still, Dr Embarek said that further studies should be conducted to ascertain if frozen wild animals - under the right conditions - could spread the virus.

HOW DID THE WORLD REACT?

US National Security Adviser Jake Sullivan expressed "deep concerns" about the findings and urged China to make available data from the earliest days of the outbreak.

British Prime Minister Boris Johnson mooted a global contract on pandemics, where countries can share data on future pandemics, after his foreign minister voiced concerns about the level of access given to the WHO mission.

China has fired back, with a spokesman for the Chinese Embassy in Washington saying the US should not be pointing fingers at China and other countries that supported the WHO during the pandemic.

WHAT'S NEXT?

Following questions of whether some hypotheses have been discarded, Dr Tedros said last Friday that all possibilities are still open and require further study in the search for the origins of Covid-19.

Future studies could include how the virus infects humans from frozen food products, tracing the supply chain of frozen animal products at the Huanan market to pinpoint a possible animal host for the coronavirus and expanding the search globally to look for evidence of circulation of Covid-19 before December 2019.

A summary of the team's findings could be released as early as this week, while the full report, which includes recommendations, will be released after that.