One in three coronavirus patients are ‘silent carriers’ testing positive but showing NO symptoms

Classified Chinese government data suggests that the number of "asymptomatic" people may be higher than initially thought.

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Figures seen by show that, by the end of last month, 43,000 people had tested positive for coronavirus in China without showing symptoms.

These people were quarantined but not counted in official figures - which stood at 80,000 at the time.

So far, scientists have been unable to determine how infectious those without symptoms are and what effect it has on the virus' spread.

The World Health Organisation's guidance on asymptomatic transmission is that it is "extremely rare".

Challenging the numbers

In an earlier report by the WHO's international mission to China, it had estimated that asymptomatic infections accounted for one to three per cent of cases.

But in February, a group of Japanese experts - led by Hiroshi Nishiura - challenged those figures.

The epidemiologist, based at Hokkaido University, wrote a letter to the International Journal of Infectious Diseases.

It said: "The number of novel coronavirus (Covid-19) cases worldwide continues to grow, and the gap between reports from China and statistical estimates of incidence based on cases diagnosed outside China indicates that a substantial number of cases are underdiagnosed."

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Nishiura said the proportion of asymptomatic Japanese patients evacuated from Wuhan - where the virus originated - at 30.8 per cent.

This figure is in line with the newly released classified Chinese government data.

Nishiura added: "The asymptomatic ratio… could be higher among children than in older adults.

";That would considerably change our scope of the outbreak, and even the optimal interventions can change."

The WHO includes all those who test positive as confirmed cases - regardless of whether they experience any symptom.

Revised guidelines

On February 7, the Chinese government revised its guidelines to count only those patients with symptoms as confirmed cases.

In the UK - as well as other countries including the US - people are only tested if they show symptoms.

Experts now say that testing every who has had contact with a patient who has tested positive - even if they're not sick - is more efficient at stopping the spread of the virus.

In another study, scientists from the United States, France, China and Hong Kong, found that time between cases in a chain of transmission is less than a week.

Their findings, published in the journal Emerging Infectious Diseases, also showed that more than 10 per cent of patients are infected by somebody who has the virus but does not yet have symptoms.

Last week, a series of studies were released which indicate people without Covid-19 symptoms are acting as unseen “super spreaders”.

Science journal Nature says the warning sign of “covert transmission” is finding sick people with no recent international travel or contact with anyone displaying symptoms.

The exact rate of such transmission is unknown, but inferences are it’s very high.

Don't wait

Jeremy Farrar, a London-based infectious disease specialist, told Nature: “We need to make policy decisions and clinical decisions now.

“You can’t say, ‘Let’s wait a month until we have the data’.”

The dramatic outbreak in the US state of Massachusetts is a case in point.

Researchers believe at least 82 cases out of 2,000 were contracted from spreaders not yet showing symptoms.

They pointed to one key event in February, when a biotechnology company held a staff conference.

In the days after the event, three employees fell ill and tested positive for the virus.

Later, more than 70 diagnosed Covid-19 cases were traced back to the conference.

Large-scale modelling

New large-scale mathematical modelling studies have exposed a significant amount of “asymptomatic” spread within Singapore and Tianjin Province, China.

One pre-print study indicates between 48 per cent and 66 per cent of 91 cases in Singapore must have contracted the virus from a carrier not showing symptoms.

In China, the figure was much higher – some 62 per cent and 77 per cent.

Another points to people being infectious on average between 2.55 and 2.89 days before the onset of symptoms.

Research has also found that those infected with Covid-19 can take a staggering five days for coronavirus symptoms to show - and they can still appear after the quarantine period.

The study, from the Johns Hopkins Bloomberg School of Public Health in the US, found that the average incubation period is 5.1 days.

And they say that almost all  - 97.5 per cent - of those who develop symptoms appeared to do so within 11.5 days of infection.

Visualisation of how far a cough travels through the air despite using your hand, a mask or your sleeve