SCIO briefing on scientific research into COVID-19
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E-mail China.org.cn, April 29, 2020
Bloomberg:
I have two questions. The first is about the asymptomatic carriers. Is there any update concerning the way asymptomatic cases spread or about their causes? Also, there are patients in many places who have developed symptoms after 14 days of isolation. Does this indicate that the incubation period of the novel coronavirus is longer than 14 days? Thank you.
Wang Guiqiang:
This is also a problem concerning epidemiology. First of all, there are indeed asymptomatic carriers both in China and the rest of the world. Currently, many of the imported cases in China are asymptomatic carriers. Statistics from the U.S. also show that random testing in the street has found many asymptomatic carriers. There are two groups of people among asymptomatic carriers. The first group are those who tested positive in the nucleic acid tests whilst still in incubation period, thus haven't yet developed any COVID-19 symptoms. As time goes by, some of them will develop symptoms. Therefore, these people are actually typical cases in incubation period, rather than asymptomatic carriers in a true sense. However, currently we are still not able to distinguish them, thus they are categorized as asymptomatic carriers.
The second group are asymptomatic carriers in a true sense. This is a common feature of epidemics. While the epidemic rages, many patients get the virus but develop no symptoms, which is called a subclinical infection. Thus, these patients don't go to hospitals for medical treatment, but when we take a look back at the end, we may find that these patients were indeed infected. These two groups are similar in terms of the physical performance.
The main problem of an asymptomatic infected person is actually the problem of contagion. We believe that asymptomatic carriers are clearly infectious, and they should be treated equally with other COVID-19 patients. The infectivity during the incubation period is relatively stronger, and the viral load of asymptomatic carriers is generally lower. Still, this is only a relative number, and it cannot represent the relationship between infectivity and non-infectivity. Therefore, a part of asymptomatic carriers may appear as a latent infection, meaning they will become patients later. Thus, isolated observation is very important.
So far as the asymptomatic carriers can only be detected by means of testing, China has made it very clear that the role of large-scale nucleic acid testing should be much strengthened with the hope that those asymptomatic infections can be discovered as soon as possible. At present, we do routine screening for those who come back to China from outside the country, and it is precisely this routine screening that has found so many asymptomatic carriers. We also conduct general screening of the general population and high-risk groups, with the aim of finding asymptomatic infected persons, and then isolating and observing them in time to avoid further transmission. Thank you.
Jin Qi:
Professor Wang has made some very marvelous remarks while answering those questions. I would like to add a few of words concerning the onset of the disease after the 14-day quarantine. My general point of views consists of two aspects. First, the cases are quite rare, and, second, they are normal. Why should I hold such views? Because people differ greatly in routes of transmission and degrees when they have been infected by the disease. More to the point, for each individual, the infection routes, immune systems and body reactions are different. All those factors combined lead to a variety of incubation periods, say, one, three or ten days. It also leads to different symptoms, such as diarrhea or fevers, tested at 37, 37.5, 38 or 39 degrees Celsius. I see nothing unusual from this point of view that an incubation period can still run over 14 days, though, in accordance with the statistics and dynamic layouts, the occurrence is extremely rare. Therefore, a quarantine for a given incubation period of 14 days is completely rational. Thank you.
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