On January 27 2020, Germany detected its first known Covid case - but the diagnosis surprised doctors and scientists.

The German businessman had been tested after coming into contact with a colleague visiting from China who subsequently fell ill, but at the time of her stay she had seemed perfectly healthy: no coughing or sneezing or complaints of illness over two days of meetings.

She had been back home in Shanghai for days before she tested positive.

In an interview with the New York Times back in June 2020, Dr Camilla Rothe - whose patient was Germany's Case Zero - recalled that at the time it was the scientific consensus that only those with symptoms could spread the virus.

"People who knew much more about coronaviruses than I do were absolutely sure," she said.

Months later it emerged that the Chinese worker had in fact felt fatigued and experienced muscle pain during her stay.

Nonetheless, Dr Rothe and her colleagues were among the first in the world to raise the alarm over the potential for asymptomatic transmission which, if widespread, spelled disaster for efforts to contain the disease.

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When a meeting of the UK Government's Scientific Advisory Group on Emergencies (SAGE) met on January 28 2020, they noted that "limited evidence" of asymptomatic transmission was beginning to emerge.

That same day, according to evidence led at the UK Covid inquiry, England's chief medical officer Professor Sir Chris Whitty said he had pulled aside then-health secretary, Matt Hancock, to inform him that there was "credible evidence" from Germany that the virus which causes Covid could be silently spread by people with no symptoms.

The problem appears to have been widely discussed and evaluated during February in a number of the government's scientific and medical advisory bodies, and yet - according to his testimony this week to the UK Covid inquiry - Boris Johnson was blissfully unaware of the danger.

The Herald: Matt Hancock, UK health secretary during the pandemic, said there was a 'protective ring' around care homes at a time when untested patients were being transferred from hospitalsMatt Hancock, UK health secretary during the pandemic, said there was a 'protective ring' around care homes at a time when untested patients were being transferred from hospitals (Image: PA)

Had he known, said the former Prime Minister, the "panic level would have been much higher".

"The information that I was getting right up to the middle of March was that you were unlikely to have Covid unless you had symptoms," he added.

It is up to Baroness Heather Hallett to draw her own conclusions.

Was there really such an important breakdown in communication, or was the emerging reality simply dumped in the 'too difficult' pile given that the UK's testing capacity at the time could not even keep pace with demand from symptomatic cases?

To paraphrase, "recollections may vary".

According to his own testimony at the end of November, Matt Hancock insists that it is "not fair to say that the scientists knew about this [asymptomatic transmission] for sure by mid-March".

Yet he also said it was his "biggest regret" that he did not overrule the scientists and demand blanket testing in high risk circumstances such as the transfer of hospital patients into care homes - something eventually introduced in mid-April, by which time tens of thousands of elderly people had already been discharged, untested, to free up beds in the NHS.

Dominic Cummings - one of Mr Hancock's most exuberant critics - told a different story; in his version, the health secretary "kept repeating this false idea" well into March that tests could not detect asymptomatic Covid cases despite being told by the government's chief scientific advisor, Sir Patrick Vallance, that this was plainly wrong.

Such misinformation had "badly confused" the Prime Minister, said Mr Cummings.

The Herald: Dominic Cummings accused former health secretary Matt Hancock of misunderstanding the science on asymptomatic CovidDominic Cummings accused former health secretary Matt Hancock of misunderstanding the science on asymptomatic Covid (Image: PA)

Looking back, it is clear that asymptomatic transmission was being flagged as a risk - but it was less clear exactly how much of a risk it posed.

A Lancet study on February 24 2020 described that "infected individuals can be infectious before they become symptomatic".

NHS guidance issued to clinicians on March 3 recommended that persons who were asymptomatic but had a coronavirus travel history or close contact with confirmed case should be "advised to stay indoors [and] avoid contact with other people".

On March 5, Prof Whitty - under questioning from MPs - said there was a "large iceberg of people who have asymptomatic transmission".

On this basis at least, it is difficult to see how the hospital-to-care home 'no testing' policy could have been waved through as safe.

The uncomfortable truth appears to be that it was simply expedient; that on a balance of risks, at a time when a worst-case scenario anticipated hospitals becoming overwhelmed with Covid admissions, the priority was to free up space for younger adults at the potential expense of the frail elderly.

Even if subsequent investigations, including those by Public Health Scotland, have concluded that the virus entered care homes primarily via other routes - such as infected staff - that does not mean the risk was not taken.

Likewise, social care was left to muddle on in the early days with little to no PPE supplies when it was known that asymptomatic carers could be infecting their patients.

Asymptomatic spread of Covid was, and is, one of its most pernicious traits.

The original SARS virus, which emerged in China in 2003, was much deadlier but it succeeded in spreading to just four other countries before being stopped. Infected patients only became contagious once they were symptomatic, making it much easier to test, trace, and quarantine.

When its cousin SARS-CoV2 - the coronavirus behind Covid - emerged in 2019, the initial assumption was that it would behave in the same way.

Exactly how big a problem asymptomatic transmission is is still difficult to quantify.

As late as June 2020, the World Health Organisation's Dr Maria van Kerkove - its Covid-19 technical lead - caused controversy by saying asymptomatic transmission may be a "rare event".

Estimates for the percentage of people who experience Covid asymptomatically throughout the course of their infection have varied from 5% to 80%. More recently it has been put at around 44%.

What percentage of these individuals are actually infectious is even trickier to gauge.

A Stanford University study in April this year - based on analysis of samples from 242 PCR-positive but asymptomatic patients in hospital - concluded that at least three quarters were not currently infectious.

This is complicated further, however, by the fact that PCR tests can pick up residual virus weeks after patients have recovered.

Nearly four years after it first emerged, Covid continues to confound us.