Reflection on lower rates of COVID-19 in children: Does childhood immunizations offer unexpected protection?

The incidence of COVID-19 in children and teenagers is only about 2% in China. Children had mild symptoms and hardly infected other children or adults. It is worth considering that children are the most vulnerable to respiratory pathogens, but fatal SARS-like virus had not caused severe cases among them. According to the pathological studies of COVID-19 and SARS, a sharp decrease in T lymphocytes leads to the breakdown of the immune system. The cellular immune system of children differs from that of adults may be the keystone of atypical clinical manifestations or even covert infection. The frequent childhood vaccinations and repeated pathogens infections might be resulting in trained immunity of innate immune cells, immune fitness of adaptive immune cells or cross-protection of antibodies in the children. Therefore, due to lack of specific vaccine, some vaccines for tuberculosis, influenza and pneumonia may have certain application potential for the front-line health workers in the prevention and control of COVID-19. However, for high-risk susceptible populations, such as the elderly with basic diseases such as hypertension and diabetes, it is necessary to explore the remedial effect of the planned immune process on their immunity to achieve the trained immunity or immune fitness, so as to improve their own antiviral ability.