Age trend
A survey report on the epidemiological characteristics of COVID-19 cases from January 21 to February 20, 2020 in Zhejiang Province,China, indicated that in 108 asymptomatic cases, patients <30 years old accounted for 28.70%, higher than the ratio of the confirmed cases(13.86%)[6] .Another investigation of imported cases from March 1 to April 7 from the same place showed that 43 cases were asymptomatic carriers in total 91 cases reported(47.25%), and the proportion of patients < 30 years old in asymptomatic cases was 51.16%, while it was 33.33% in the confirmed cases [7]. In Hunan province, China, the median ages of the confirmed cases and asymptomatic carriers of COVID-19 reported were 44 and 35 years respectively, and the proportion of young patients aged 3-18 years was proved to be relatively higher in asymptomatic cases ( χ2= 65.52, P<0.01)[8]. From January 21 to February 29, there were 14 cases of asymptomatic COVID-19 infection in Yangzhou, Jiangsu province, China, the majority of them were young adults, 85.7% of them were under 50 years old and the average age was 38.7 years[9].
Although some of the sample size was relatively small, the surveys were all from provinces located close to Hubei where the outbreak first started and the epidemic was severe, so we believed the results have some underestimated value. Moreover, We have reviewed literatures on family aggregation epidemic and list the asymptomatic carriers or close contacts without infection(no clinical symptoms and negative nucleic acid test), and found that women and children make up the majority of these cases(13/16 , 81.25%)(Table 1.)[10-21]. It is interesting that the six uninfected close contacts cases were all young females or children, none was adult male, though our sample size is very small(Table 1.). Several researches have showed that children are not susceptible to SARS-CoV-2 infection compared with the adults[22, 23], which still needs a large-scale epidemiological investigation to confirm it. We will talk about the children asymptomatic cases in this article later.