Research published last week by Jeffrey Shaman of Columbia University in New York and his colleagues analysed the course of the epidemic in 375 Chinese cities between 10 January, when the epidemic took off, and 23 January, when containment measures such as travel restrictions were imposed.
The study concluded that 86 per cent of cases were “undocumented” – that is, asymptomatic or had only very mild symptoms (Science). The researchers also analysed case data from foreign nationals who were evacuated from the city of Wuhan, where the first cases were seen, and found a similar proportion of asymptomatic or very mild cases.
Such undocumented cases are still contagious and the study found them to be the source of most of the virus’s spread in China before the restrictions came in. Even though these people were only 55 per cent as contagious as people with symptoms, the study found that they were the source of 79 per cent of further infections, due to there being more of them, and the higher likelihood that they were out and about.
A project in Italy has also found many symptomless cases. When everybody was tested in a town called Vò, one of the hardest-hit in the country, 60 per cent of people who tested positive were found to have no symptoms.
Vir: New Scientist