We read the article by Li, et al. [1] with interest and would like to offer some observations about the study based on the current literature.

In the pathogenesis of a standard viral infection, the pathogen’s contact with the mucosa is initially followed by an innate immunity response (macrophage, antigen presenting and natural killer cell). Subsequently, adaptive immunity comes into play and is responsible for the elimination of infected cells, activation of the antibody response, and production of memory T-cells. T-cells are the primary decisive element in adaptive immunity capability. For this reason, the adaptive immune response mediated by the thymus is a process that regulates the immune response responsible for preventing invasive damage from a virus. Therefore, the thymus is the most influential organ in the transmission of viral disease [2].

The thymus generally decreases in function and anatomically shrinks with age. This function and size loss becomes specially prominent after the age of 50 [3]. Thymic involution and the gradual decrease in T-cell count and ability with age are together termed as immunosenescence [4]. The primary reason for morbidity and mortality in COVID-19 cases is due to lung manifestation. The primary reason for a frequently severe clinical presentation in patients of ages 50 and up is thought to be due to a deficient, irregular and uncontrollable antiviral response as a result of thymus involution and immunosenescence. Important factors in achieving an adequate immune response are an increase in thymus activity and T-cell action along with immune system coordination.

When examining the critical COVID-19 cases in the literature, the male gender seems to be more common; this is speculated to be due to greater tobacco use and ACE-2 receptor expression. The literature also shows that thymic involution is more apparent in males compared to females. This difference in thymic involution indicates that males face a greater extent of immunosenescence. We believe this mechanism might be responsible for clinical worsening in males [5].

We believe that thymus regression and lung immuno-senescence are the main deciding factors of lung involvement depth in adult COVID-19 patients. But, we do not know thymus activity in children cases with severe COVID-19. We feel that there is a need to examine the patients for thymus size, and look for association between thymus size and the severity of lung involvement.

1. Li B,　Shen J,　Li L,　Yu C. Radiographic and clinical features of　children　with 2019 novel　coronavirus　(COVID-19) pneumonia. Indian Pediatr.　2020; 57:423-6.

2. Savino W. The thymus is a common target organ in infectious diseases. PloS Pathog. 2006;2:e62.

3. Gruver AL, Hudson LL, Sempowski GD. Immuno-senescence of ageing.　J Pathol. 2007;211:144-56.

4. Weiskopf D, Weinberger B, Grubeck-Loebenstein B. The aging of the immune system.　Transpl Int. 2009;22:1041-50.

5. Zhou F, Yu T, Du R, et al. Clinical course and risk factors for mortality of adult in patients with COVID-19 in Wuhan, China: A retrospective cohort study. Lancet. 2020;395:1054-62.