WASHINGTON (BLOOMBERG) - Of all the issues that have roiled ties between China and the West since the pandemic emerged, none has been more sensitive in Beijing than questions about the origin of Covid-19.
China last year responded to US ally Australia's initial push for an independent probe into where the virus came from with tariffs on exports of its barley and wine. Since then Beijing has repeatedly blasted calls for more transparency as politically motivated, and sought to deflect suggestions it came from a laboratory in Wuhan with alternative origin theories ranging from transmission via frozen-food imports to a release from US bio-facilities.
That made US President Joe Biden's revival of the lab theory last week - by giving intelligence agencies 90 days to get closer to a definitive conclusion on the origin of the coronavirus - all the more meaningful.
The order came as political pressure increased following a series of reports suggesting the theory, which was prominently backed by Trump administration officials, had been prematurely disregarded. Even Facebook said it would stop taking down posts claiming Covid-19 was man-made or manufactured.
The origin probe comes at a sensitive time in US-China relations, with both governments showing signs they want to move beyond the vitriol that marked the last year of Mr Donald Trump's presidency.
While Mr Biden has kept in place Mr Trump's tariffs and sanctions against Beijing, his administration has also started to open a dialogue: Treasury Secretary Janet Yellen held her first call with Chinese Vice-Premier Liu He on Tuesday (June 1) evening in Washington, following his conversation last week with US Trade Representative Katherine Tai.
Mr Biden plans to amend a US ban on investments in companies linked to China's military this week after several companies successfully challenged it in court. Still, it is unclear how the US will implement the new order targeting companies connected to China's defence and surveillance technology sectors, which could be even broader.
Either way the results of the lab leak probe could tie the hands of Chinese President Xi Jinping, particularly if comes just before he has an opportunity to meet Mr Biden at the Group of 20 summit in October.
While Mr Xi realises that China's aggressive response has hurt its standing abroad - this week he urged officials to create a "trustworthy, lovable and respectable" image for the country - the Chinese Communist Party has little choice but to vigorously resist any suggestion it mishandled or covered up a virus that effectively shut down the world.
"The issue of the origin of the virus is deeply connected to the legitimacy of the CCP, so I do not expect that China will become more transparent - it will fight this relentlessly," said Ms Bonnie Glaser, director of the Asia Programme at the German Marshall Fund of the US, a Washington-based policy research group.
"That said, China is unlikely to use the forms of economic coercion against the US that it is using against Australia, in part because it fears US retaliation in the form of further restrictions on high-tech," she added. "It also fears the downward spiral of the U.S.-China relationship to dangerous levels."
Since the early days of the pandemic, some US intelligence officials have suggested SARS-CoV-2 - the coronavirus that causes Covid-19 - may have leaked either accidentally or deliberately from the Wuhan Institute of Virology. Researchers at a high-tech lab there have studied other coronaviruses, including some that were isolated from bats.
A World Health Organisation report drafted with China earlier this year called the possibility of a lab leak "extremely unlikely" because no such virus was studied there before the outbreak and security protocols don't appear to have been breached.
The report, compiled with local researchers after Beijing finally allowed a group of WHO experts into Wuhan, said the most likely origin scenario was that the virus spread to humans from bats through an unspecified intermediary animal.
Still, the White House criticised the report as incomplete, while WHO Director-General Tedros Adhanom Ghebreyesus said it was not "extensive enough" and that the lab-leak theory needed more investigation. Other prominent Western scientists have urged that hypotheses about natural and laboratory spillovers be taken seriously until there is sufficient data to say otherwise.
While Mr Biden's move to publicly announce the virus probe appeared "purely political," Dr Tedros' criticism of the WHO report is harder for China to dismiss "and it's driving them crazy", according to Mr Christopher Johnson, a former senior China analyst at the Central Intelligence Agency who is now a senior fellow at the Centre for Strategic and International Studies. Either way, he added, China is unlikely to allow any more access that would shed further light on the origin.
"Finding a smoking gun on this would be nearly impossible, so the only way you could deescalate is to actually be talking to each other," Mr Johnson said. "Without that, the potential to inflame the relationship further is there."
Chinese diplomats and senior officials at the Wuhan lab have repeatedly denied that the facility was the source of the coronavirus. Responding to the Biden probe, Foreign Ministry spokesman Zhao Lijian last week pointed to the WHO findings while questioning why US intelligence agencies were getting involved.
"How can anyone trust the findings from an 'investigation' conducted by such an intelligence organ with no credibility to speak of?" he said on May 27.
Mr Zhao also pointed to "suspicion-shrouded" Fort Detrick in Maryland, a base that is home to the American bio-defence agency. This was a repeat of a theory Mr Zhao has long pushed that the virus may have originated in the United States.
Chinese officials have emphasised research showing evidence of coronavirus infections outside China in late 2019 and pointed to frozen food imports as a possible vector - all part of efforts to cast doubt that the virus originated in or around Wuhan, the central Chinese city that saw the first known Covid-19 cases.
Mr Biden's intelligence directive now ensures the discussion will continue for at least the next several months, and US-China ties could get even worse if the probe ends up fingering Chinese officials, according to Mr Hugo Brennan, an analyst at risk consulting firm Verisk Maplecroft.
"If the US intelligence community concludes that the 'Wuhan lab leak' theory is the most likely cause of the pandemic, it will see US-China relations plummet to new lows," he said. "Particularly if the consensus is that Beijing deliberately tried to cover up the leak."