BACKGROUND: Severe cases of coronavirus disease 2019 (COVID-19) among pediatric patients are more common in children less than 1 year of age. Our aim is to address the underlying role of immunity and inflammation conditions among different age groups of pediatric patients. METHODS: We recruited pediatric patients confirmed of moderate COVID-19 symptoms, admitted to Wuhan Children's Hospital from January 28th to April 1st in 2020. Patients were divided into four age groups (</= 1, 1-6, 7-10, and 11-15 years). Demographic information, clinical characteristics, laboratory results of lymphocyte subsets test, immune and inflammation related markers were all evaluated. RESULTS: Analysis included 217/241 (90.0%) of patients with moderate clinical stage disease. Average recovery time of children more than 6 years old was significantly shorter than of children younger than 6 years (P = 0.001). Reduced neutrophils and increased lymphocytes were significantly most observed among patients under 1 year old (P < 0.01). CD19+ B cells were the only significantly elevated immune cells, especially among patients under 1 year old (cell proportion: n = 12, 30.0%, P < 0.001; cell count: n = 13, 32.5%, P < 0.001). While, low levels of immune related makers, such as immunoglobulin (Ig) G (P < 0.001), IgA (P < 0.001), IgM (P < 0.001) and serum complement C3c (P < 0.001), were also mostly found among patients under 1 year old, together with elevated levels of inflammation related markers, such as tumor necrosis factor gamma (P = 0.007), interleukin (IL)-10 (P = 0.011), IL-6 (P = 0.008), lactate dehydrogenase (P < 0.001), and procalcitonin (P = 0.007). CONCLUSION: The higher rate of severe cases and long course of COVID-19 among children under 1 year old may be due to the lower production of antibodies and serum complements of in this age group.