IT was described by a World Health Organisation scientist this week as the “big open question” of the pandemic: how many cases of coronavirus are being spread by people showing no symptoms? Who might never develop any symptoms.

The issue has important implications for Scotland’s test and protect regime, as it does for the rest of the track, trace and isolate measures now in place across the rest of the UK.

Contact-tracing and containment of the virus post-lockdown is built around the idea that anyone in the community displaying Covid symptoms can now be tested, and their recent contacts identified and asked to self-quarantine as a precaution.

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But the process will never be triggered in the case of someone unwittingly passing the virus on who never experiences any signs of being sick. Or who develops atypical symptoms not normally associated with Covid. Or is so mildly ill that they barely notice.

So how big a problem are these people anyway?

A small scientific furore was sparked on Monday when the WHO’s Covid-19 technical lead, Dr Maria Van Kerkhove, remarked that such transmission was “very rare”.

The American epidemiologist, who is an expert in high-threat pathogens, backtracked 24 hours later - saying it had been a misunderstanding.

She stressed that, in fact, modelling studies suggest that up to 40 per cent of coronavirus infections could be transmitted by people without symptoms - although it could also be as low as 4-5%.

Her “very rare” comment had related to a small subset of studies and data shared with the WHO that is not yet published.

Dr Van Kerkhove said this particular evidence came from countries that had carried out “detailed contact tracing”. Where an asymptomatic case had been detected in the population and followed up, she said, it was “very rare” to find secondary infections among their contacts.

But she it was still a “big open question” as to whether the same was true globally.

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There was no doubt that asymptomatic transmission happened, she said, but how often was still one of the great Covid mysteries scientists are trying to unravel.

“We don’t have the answer yet,” she said.

Director of the WHO’s health emergencies programme, Dr Michael Ryan, said he was “absolutely convinced” asymptomatic transmission was occurring, “the question is how much”.

It is not necessarily unusual for people with viral infections to show no symptoms.

A 2018 study by public health experts from Columbia University in the US randomly tested 2,685 people visiting a New York tourist attraction, finding that 168 tested positive for at least one common respiratory virus. Of these, 65 to 97% did not report any symptoms.

And in May, the same researchers published a study in the journal, Science, which estimated that undocumented Covid-19 cases - that is, those with very mild or no symptoms - were responsible for causing 79% of the documented cases in China prior to travel restrictions coming into force on January 23.

One particular case marked a turning point in our understanding of the public health threat posed by Covid.

On January 19, worshippers who gathered at the Life Church and Missions in Singapore were joined by a Chinese couple who had flown in that morning from Wuhan.

They appeared perfectly healthy, with no cough - then considered the main symptom of Covid.

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The couple left as soon as the service was over, but three days later on January 22 the wife began showing symptoms followed by her husband on January 24.

Meanwhile, three local people back in Singapore also fell ill. Contact tracers quickly linked the outbreak to the church and established that two of the Singaporeans who were infected had been at the same service as the Chinese couple.

The third Singaporean to become infected, a 52-year-old woman, was more puzzling. She had attended a separate service hours later on the same day - but CCTV footage later revealed that she had sat in the same seats that the couple had used.

It is now thought that most people with Covid become contagious three days before showing symptoms, although it remains unclear exactly how they pass it on during this phase as droplets - for example, from coughing - are still considered the main source of transmission.

Scientists at the Earlham Institute in Norwich are currently pushing for all of the city’s 132,000 inhabitants to be tested to help gauge the prevalence of asymptomatic carriers, something lead researcher Professor Neil Hall has described as “the ‘dark matter’ of the epidemic”.

“Any intervention that’s only based on people coming to primary health care when they have symptoms will only deal with half the problem,” he said.

If so, random and routine population testing may be the next step UK-wide.