Children asymptomatic cases
A report from the Wuhan Children’s Hospital which located in the outbreak and worst-hit areas
described the spectrum of SARS-CoV-2 infection in children, it analyzed 171 cases and concluded that most infected children appear to have a milder clinical course in contrast with infected adults, and asymptomatic cases(27/171, 15.8%)were more common[43]. A retrospective study of 2143
pediatric patients infected with SARS-CoV-2 in China come to a similar conclusion, the majority of the patients were asymptomatic(94, 4.4 %), mild (1091, 50.9 %), or moderate cases( 831, 38.8 %), totally accounted for 94.1 % of all cases. It suggests that there are fewer severe cases but more asymptomatic infections in children compared with adult patients[44].
Some scholars believe that this is due to less immune dysfunction after the SARS-CoV-2 infection in children[45-48]. It is very meaningful that vaccination with the tuberculosis vaccine bacillus Calmette Guerin (BCG) could dramatically reducing infant deaths from sepsis and respiratory tract infection (e.g. influenza). BCG has also been proved to protect mice from influenza virus infection via nonspecific protection in animal experiment[49]. This is a type of immune memory phenomenon that is unique to innate immunity after body is infected or vaccinated and it can induce T cell-independent protection against secondary infection, we call it “training immunity”[50]. Perhaps the helpful training immunity in asymptomatic or mild infection children could contribute to the future evolution of immunotherapy and vaccines for the SARS-CoV-2. Another speculation of why children were less sensitive to SARS-CoV-2 was that the function (e.g. binding ability) of Angiotensin Converting Enzyme-2 (ACE-2) identified as the cell entry receptor of SARS-CoV-2 in children may be lower than that in adults[44].