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.