The proportion of asymptomatic infections
The epidemic of SARS-CoV-2 infection is spreading worldwidely, compeling us to live with the virus for perhaps a long time. From the prevention point of view, the asymptomatic carriers are the most difficult to control, because they spread the virus unknowingly on some occasions. Estimation of the prevalence of asymptomatic virus shedding such as asymptomatic ratio by using target populations could improve understanding of COVID-19 transmission and provide vital insight into the epidemic spread[51]. The figure can put severe cases and mortality rates into an accurate context so that the fatality rate and the proportion of patients requiring hospitalization would be estimated accurately, which is critical to balancing the socio-economic burden of infection control interventions against their potential benefit for humans[52]. As of June 06, 2020, 7,938 asymptomatic infections had been confirmed on the mainland of China, including 1,358 re-categorized as confirmed cases, while total confirmed cases reached 83036[53]. At present, the proportion of asymptomatic infection in mainland China was 7.23% (6580/90974), seems lower than our estimation, which may be attributed to the following factors: incomplete screening, defective detection methods, and some patients have conscious symptoms due to psychological pressure and so on.
How many asymptomatic cases globally, it’s hard to answer this question now. Japanese scholar estimated that the asymptomatic proportion of COVID-19 on board the Diamond Princess cruise ship was at 17.9% (95%CI: 15.5–20.2%) by conducting statistical modeling analyses[54]. Another assessment of the asymptomatic ratio among Japanese nationals who evacuated from Wuhan, China, got a higher outcome of 41.6% (95% CI: 16.7%-66.7%). But the sample data of both studies that adopted in the analysis did not constitute a random sample of a general population, and there was a bias from the presence of underlying diseases or comorbidities of the samples, which brought some limitations. Taking results from several studies into account, Gerardo Chowell, a mathematical epidemiologist at Georgia State University in Atlanta, thinks that “asymptomatic or mild cases combined represent about 40–50% of all infections”[55]. To assess the extent of hidden infection, a group of researchers had developed a model using clinical data from 26,000 laboratory-confirmed cases reported to the health commission of Wuhan, the epicentre of the outbreak, China. “By our most conservative estimate, at least 59% of the infected individuals were out and about, without being tested and potentially infecting others,” says Wu Tangchun who in charge of the project[55]. We look forward to the final detailed results yet not published.