This study used network analysis to explore the comorbidity network of depression, anxiety and stress among 887 Chinese citizens during the COVID-19 pandemic. In the network, we determined the network structure and the expected influence and bridge expected influence of each node. The network structure shows that many symptoms (128/210) in the network are directly connected, which is consistent with the results of previous studies [30, 31]. Moreover, the network structure shows that there are many correlations between stress and depression and anxiety symptoms. We found that the edges of symptoms motor -anhedonia and hopeless-meaningless have the strongest edge weight, followed by edge weight of symptoms dyspneic-heart sick, worthless-meaningless and over reactive-touchy. These results indicate that the connection of edges within factors in the network has a greater connection than the connection of edges between factors, consisting with previous research results [19, 32]. This study also found that the strongest edge of the relationship between disorders is the stress symptom uneasy and depression symptom sad mood. People under long-term stress may weaken bodys’ immune response-ability, especially acute stress causes sympathetic-adrenaline-mediated increase, and induces chemotaxis and adhesion molecule expression, thereby promoting immune cell migration to infection or inflammation, while chronic stress impairs this mechanism . Inflammation plays a key role in the pathogenesis of depression. Larger, more frequent or longer-term inflammatory reactions may have a negative impact on mental and physical health , meanwhile chronic Low-level inflammation can also trigger mental and health problems. These evidences indicate that both acute stress and continuous chronic stress during the COVID-19 pandemic may trigger an inflammatory response, reduce a body's immunity and increase risks of depression and infecting COVID-19. Our results support that stress is closely associated with depression, not just anxiety closely associated with it [35, 36].
The results of this study show that depression symptoms blue and stress symptoms relax and intolerable have the strongest expected influence, which indicates that in COVID-19 pandemic due to the influence of social isolation, depression-related symptom blue and stress-related symptoms relax and intolerable become the most important symptoms in the present symptoms network. The intervention of these symptoms could have a good effect to reduce the severity of overall symptoms in the netowork. However, in previous studies depression symptom sad mood [19, 37] and depression symptom fatigue  have the strongest centrality. Similar to the network results of depression, anxiety, and traumatic stress disorder, symptom feeling blue has the greatest strength in this network . In this study, most notably stress symptoms relax and intolerable have the strongest centrality. The possible reason is that this study was conducted when there were the largest number of diagnosed patients with COVID-19 pandemic in China (the information from the official website of the National Health and Health Commission). The increasing number of infections and deaths has increased people's pessimistic experiences and stress feelings. The physical symptoms of anxiety disorder, dry and trembling, are the least important symptoms in this network, although some studies have shown that some physical symptoms coexist with anxiety and depression . Network analysis studies also show that physical symptoms such as finger tingling and muscle pain are the least important symptoms . This may indicate that physical symptoms are not important nodes in similar depression and anxiety networks.
Three symptoms with the highest bridge expected influence are stress symptom intolerable and depressive symptoms sad mood and blue which is different from the previous research results --- the dysphoria/avoidance-related symptom  and the anhedonia, sad mood and fatigue symptoms  showing high bridge centrality values. This means that stress symptom intolerable during the CVOVID-19 pandemic is a key node for activating depression and anxiety. Previous network studies did not involve stress variable, but in the context of COVID-19 pandemic, the stress symptom intolerable has the strongest bridge expected influence, indicating that stress symptom intolerable is a significant node during the COVID-19 pandemic. That means stress is a crucial research variable in public health emergencies. Therefore, future research should further explore the co-occurrence of stress, depression and anxiety symptoms.
The prevention of bridge symptoms can block the spread of comorbidities, and it is more effective to eliminate the bridge symptom nodes than do on traditional central nodes . The outbreak of COVID-19 pandemic severely impaired mental health, leading to psychological problems such as stress, anxiety and depression [41, 42]. The bridge symptom results of this study indicate that symptoms intolerable, sad mood and blue are the optimal intervention targets in order to better prevent these mental health problems. Symptom intolerable is connected to many nodes in the network, especially having the strongest connection with symptom blue, meanwhile blue and sad mood have the strongest connection, indicating that these three symptoms play the most important connection role in the comorbid network. These results support studies of mechanisms related to depression and stress. Symptom sad mood is closely related to abnormal brain connectivity in depression patients. It induces increased activation of the ventrolateral thalamus nucleus , while the abnormality of the ventral thalamus nucleus is the important abnormal brain regions of GRI model mice . Besides depressive symptoms, stress symptom intolerable is the most important factor. Research shows that intolerable (intolerance of Uncertainty) is not only significantly related to stress , but also exacerbates people’s anxiety . The results of this study provide some support for targeted interventions. During the COVID-19 pandemic, for symptoms sad mood and blue, people should do some exercise activities especially before bedtime, listen to music, have a bath or read a book to relax or do other things making themselves happy ; For symptom intolerable, it may be alleviated by obtaining pandemic information, such as current situation or treatment, or taking special preventive measures, such as paying attention to hygiene and wearing masks .
It is the first time to explore Chinese people's depression, anxiety and stress during the COVID-19 pandemic with an approach of network analysis. Our data involve 887 citizens from 31 cities in China. The network results have high stability (CS = 0.75) and are very representative. Some limitations must be considered in this study. First, the data in this study is cross-sectional, so the results of the analysis cannot determine whether these symptoms will change dynamically with time. Future research should consider whether the degree or types of the psychological problem caused by COVID-19 pandemic at different stages is different, which can help us provide more targeted interventions at different stages of the pandemic. The second is that the data only include people with college or above education background, and the result cannot be extended to a larger group. In addition, network analysis methods also have certain limitations. Network analysis is mainly a network constructed by correlation coefficients rather than causal relations between symptoms. Finally, a network constructed by network analysis is limited by collected symptoms and behaviors. For example, if other physical symptoms are added to the network, there may be different networks.