In December 2019, COVID-19 has been outbreaks in a series regions of China and abroad. Its pathogen was initially identified as a new coronavirus with single strand positive RNA. COVID-19 is considered to be a coronavirus associated with severe acute respiratory syndrome (SARS) and Middle East respiratory syndrome (MERS), which can affect the lower respiratory tract and manifest as pneumonia. On January 31, 2020, the World Health Organization (WHO) officially announced that COVID-19 has become a "public health emergency of international concern" Although most provinces in China have inhibited the spread of COVID-19 after taking protective measures, the number of patients worldwide is still on the rise. According to WHO and recent reports, On 1st Sept, 2020, the total number of clinically confirmed cases in the world is 25327098, and the current total number in China is 90402. China has incorporated COVID-19 into the class B infectious diseases in the law on the prevention and control of infectious diseases. Notably, it adopted the prevention and control of class A infectious diseases. It is worth noting that residents of Hong Kong have also been seriously infected with COVID-19. According to the latest report released by the Department of Health in Hong Kong, the total number of patients with COVID-19 had risen to 4823 on Sept 1st, 2020. Therefore, it is very urgent to find effective control methods.

According to the theory of Chinese medicine, COVID-19 belongs to the category of "epidemic toxin". TCM has been used for thousands of years in the treatment of epidemic diseases. Through the long-term struggle with the epidemic, investigators have accumulated a lot of experience for the pathogenesis, transmission, prevention and control of the epidemic. In mainland China, TCM plays an important role in the treatment of COVID-19. The recent retrospective analysis of 52 cases by Zhang Boli's research group showed that 34 cases in the treatment group of integrated TCM and Western medicine were superior to 18 cases in the simple western medicine group in terms of various bio-indexes, including clinical symptoms disappearance, the time of body temperature recovery, the average length of stay in hospital, the score of TCM Syndrome Scale, the rate of CT image improvement, the rate of clinical cure, and the conversion from common type to severe type. According to a case study of 799 people in Hunan Provincial Administration of traditional Chinese medicine, the average length of stay in hospital of integrated traditional Chinese and Western medicine treatment is 2 days shorter than that of Western medicine treatment alone, and the number of patients with obvious improvement of symptoms accounts for 64% of the total number of observation. For the modern measures of epidemic prevention, it includes three approaches: cutting off pathogens, controlling transmission routes, and reducing susceptible populations. However, so far, the preventive measures of Chinese medicine for COVID-19 has not been included in the medical system of Hong Kong, although it is widely authorized and promoted in mainland China. Therefore, it is of great clinical significance to further develop the prevention of COVID-19 by Chinese medicine for reducing susceptible populations. The National Health Committee and the State Administration of TCM organized relevant experts to establish COVID-19 standardized diagnosis and treatment program. The updated diagnosis and treatment program describes the relationship between the dialectical classification of Chinese medicine and the stage of Western medicine. The pathogenesis of TCM can be summarized as "dampness, heat, toxin, blood stasis, deficiency". According to the statement of Tong Xiaolin, COVID-19 belongs to "cold dampness epidemic", which is caused by cold dampness epidemic virus. The disease is mainly caused by the deficiency of either yin or Yang. According to a report on the clinical epidemiology of COVID-19 published by Lancet, the median susceptible population of COVID-19 is 49 years old (interquartile range is 41-58 years old), which means the majority of patients are middle-aged and elderly. The body constitution of this kind of patients is mainly divided into " Deficiency of Qi and Yang " and "Deficiency of Qi and Yin". However, so far, the prevention of treatment of COVID-19 with TCM has not been included in the medical system of Hong Kong, although TCM therapy is widely authorized and promoted in mainland China. Therefore, It may be beneficial to provide TCM preventive measures to COVID-19 in Hong Kong. By using "Self-test for classification and judgment of body constitution by TCM theory" issued by Professor Wang Qi, a honorary Professor of School of Chinese medicine, the University of Hong Kong, investigators can identify the subjects with "Deficiency of Qi and Yang " or "Deficiency of Qi and Yin". According to the above-mentioned pathogenesis characteristics of COVID-19 in the prevention stage, "Invigorating Qi and Yang, invigorating Qi and Yin" may be conducted by TCM treatment. The changes of body constitution will be measured by questionnaires before and after TCM intervention, including Self-test for classification and judgment of body constitution by TCM theory, Questionnaire of TCM symptom, and Fatigue scale.

In order to scientifically evaluate the preventive effect of TCM on COVID-19, it is very important to reveal the changes of COVID-19 related biochemical indexes before and after the intervention of TCM. According to the discussion on 《Diagnosis and treatment of COVID-19》issued by the National Health Commission, the following changes in biochemical indicators are closely related to the occurrence of COVID-19. Firstly, the decrease of leukocytes, lymphocytes, and bone marrow cells (including granulocytes, erythrocytes, and megakaryocytes) in blood routine examination; the increase of troponin and ESR. Secondly, the reduction of macrophages-related immune cells, such as monocyte. Thirdly, the increased index of liver function and cardiac function, including AST, ALT, and LDH. Fourthly, inflammation related factors, including IL-6 and C-reactive protein. Therefore, the analysis of the relationship model of TCM syndrome and molecular biological profiles can further provide a novel preventive mode for COVID-19.