Asymptomatic carriers are those who demonstrate no clinical symptoms like cough or fever but test positive for COVID-19.
According to the NHC, 1,541 asymptomatic carriers in China had been under medical observation and 205 of them were imported cases as of March 30.
Existing studies show that the proportion of asymptomatic carriers is around 20%-60%. Between January 29 and 31, Japan evacuated 565 Japanese nationals from Wuhan. Among them, 8 were infected, of which 4 had no symptoms. The asymptomatic proportion was estimated to be between 37.5%-50%. On March 6, an article, without peer review, concluded that the share of asymptomatic cases was 17.9 % from the data of passengers aboard the Diamond Princess cruise ship. On March 25, another study analyzing 115 infected children found that the share was 53%. On March 20, an article in The Nature argued that 30%-60% of all cases showed no or mild symptoms.
There are four approaches to identify asymptomatic carriers in mainland China:
proactively testing close contacts during their medical observation period;
proactively testing people during investigation of infection clusters;
proactively testing those who have been exposed to the virus during infection source tracing;
proactively testing those who have travelled to or resided in regions with ongoing COVID-19 transmission.
In an interview, Academician Zhong Nanshan argued that since the number of newly confirmed cases is decreasing in China, the number of asymptomatic carriers should be limited. Otherwise, the number of newly confirmed cases would have been pushed higher due to silent transmission.
In response to the challenges posed by asymptomatic carriers, China will scale up screening of asymptomatic carriers of COVID-19 and conduct research and epidemiological analysis through gathering a certain percentage of asymptomatic samples from the hard-hit areas, so as to improve prevention and control measures.