The number of coronavirus-infected people who show no or delayed signs could be as high as one-third of those who test positive for the disease.
The true scale and the hidden number of these’ silent carriers’ could be higher than first thought according to confidential Chinese government data seen by the South China Morning Post.
It said that by the end of February, more than 43,000 people in China had tested positive for coronavirus without symptoms, and were quarantined but not included in official figures, which at that time stood at 80,000.
This result has important consequences for the countries ‘ strategies for containing the virus.
As present, scientists are unable to agree on what part asymptomatic transmission plays in spreading the disease and how infected people are without symptoms.
A increasing number of scientists are now questioning the earlier statement of the World Health Organization that “extremely rare” asymptomatic transmission.
The earlier report by the international mission of the WHO to China had reported that asymptomatic infections accounted for 1-3% of cases.
In February a group of Japanese experts led by Hiroshi Nishiura, an Hokkaido University epidemiologist, wrote a letter to the International Journal of Infectious Diseases.
Nishiura 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.’
Nishiura put the proportion of asymptomatic Japanese patients evacuated from Wuhan at 30.8 per cent, a figure similar to the 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 counts all people who test positive as confirmed cases regardless of whether they experience any symptoms, as does South Korea.
The Chinese government revised its guidelines on the 7th February, to count only those patients with symptoms as confirmed cases.