Traditional Chinese medicine against COVID-19: Role of the gut microbiota

Coronavirus disease 2019 (COVID-19) is an acute respiratory infectious disease caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), and it has become a public health concern worldwide. In addition to respiratory symptoms, some COVID‑19 patients also show various gastrointestinal symptoms and even consider gastrointestinal symptoms to be the first manifestation. A large amount of evidence has shown that SARS-CoV-2 infection could disrupt the gut microbiota balance, and disorders of the gut microbiota could aggravate the condition of COVID-19 patients. Therefore, maintaining the gut microbiota balance is expected to become a potential new therapeutic target for treating COVID-19. Traditional Chinese medicine (TCM) has significant effects in all stages of the prevention and treatment of COVID-19. It can adjust the gut microbiota and is an ideal intestinal microecological regulator. This review summarizes the advantages and clinical efficacy of TCM in the treatment of COVID-19 and expounds on the relationship between TCM and the gut microbiota, the relationship between COVID-19 and the gut microbiota, the mechanism of gut microbiota disorders induced by SARS-CoV-2, the relationship between cytokine storms and the gut microbiota, and the role and mechanism of TCM in preventing and treating COVID-19 by regulating the gut microbiota to provide new research ideas for TCM in the prevention and treatment of COVID-19.


Introduction
In late December 2019, several health institutions reported patients with unexplained pneumonia related to epidemiology in Wuhan, Hubei Province, China. On January 7, 2020, Chinese scientists isolated the pathogenic coronavirus for the first time from respiratory tract samples of patients in Wuhan and sequenced the full length of the genome [1]. On February 11, the International Committee on Taxonomy of Viruses (ICTV) and the World Health Organization (WHO) named the virus and the disease that it caused severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and coronavirus disease 2019 (COVID- 19), respectively [2,3]. COVID-19 is extremely harmful, with the characteristics of rapid transmission, strong pathogenicity, and poor prognosis. On March 12, the WHO declared that the outbreak of COVID-19 constituted a global "pandemic" [4]. Because of the lack of drugs targeting SARS-CoV-2 specifically, as of February 26, 2022, the cumulative number of COVID-19 cases globally has surpassed 433 million in 216 countries, areas, and territories and more than 5.96 million confirmed deaths [5]. Although significant progress has been made in clinical and vaccine research, many countries are still experiencing or have experienced a second or third wave of the COVID-19 pandemic, mainly due to mutations of SARS-CoV-2 [6,7].
The initial symptoms of COVID-19 patients are mainly characterized by fever, fatigue, and a dry cough. Clinical studies have found that SARS-CoV-2 can also cause digestive symptoms, such as diarrhoea, abdominal pain, nausea, and vomiting [8][9][10][11]. Clinical data have shown that a considerable proportion of COVID-19 patients have symptoms related to intestinal flora disturbance, indicating that the SARS-CoV-2 virus could cause the problem of intestinal microecological imbalance, especially in critically ill patients with poor prognoses [11,12]. Compared with non-COVID-19 patients, the composition of the gut microbiota of COVID-19 patients has changed significantly, regardless of whether the patients have received medication [12,13]. Therefore, there could be a close relationship between the intestinal flora and SARS-CoV-2 infection [14]. The diagnosis and treatment of COVID-19 (8th trial edition) [15] clearly mentions that "intestinal microecological regulators could be used to maintain intestinal microecological balance and prevent secondary infection". Studies have shown that traditional Chinese medicine (TCM) is an effective microecological regulator that has the functions of adjusting imbalances in the intestinal flora, promoting the growth of beneficial bacteria, and inhibiting the excessive reproduction of harmful bacteria. In the prevention and control of this epidemic, TCM has been deeply involved in the whole process of prevention, treatment, and rehabilitation, complementing and cooperating with Western medicine, making important contributions and playing a unique role in the comprehensive control of the COVID-19 epidemic [16]. We performed the comprehensive retrieval of electronic databases in both Chinese and English (the China National Knowledge Infrastructure Database(CNKI), VIP Chinese Science and Technology Journal Database(VIP), WangFang Database, Chinese BioMedical Literature Database (CBM), PubMed and Embase) for collecting the published research from inception to December 10, 2021. Eventually, we summarized 69 representative clinical trials, including 19 randomized, controlled trials (RCTs), 24 retrospective cohort studies (RCSs), 10 case-control studies (CCSs), and 16 others. This review mainly summarizes the clinical efficacy of TCM in the prevention and treatment of COVID-19; the relationship between the gut microbiota and TCM; COVID-19, SARS-CoV-2, and inflammatory storms; and how TCM regulates the gut microbiota to prevent and treat COVID-19 to provide new strategies and new targets for the prevention and treatment of COVID-19.

The advantages of TCM for COVID-19
COVID-19 has the characteristics of rapid onset, strong infectivity, and easy spread. It belongs to the category of "dampness toxin epidemics" in TCM, and the main nature of the disease is as a dampness toxin. According to the theory of TCM, the disease is mainly located in the lungs and spleen. The basic pathogenesis is "dampness, toxin, heat, phlegm, blood stasis and deficiency" [17][18][19][20]. Early clinical manifestations of the disease can include fever or no fever, and fever is often accompanied by multiple symptoms of hiding fever, dry coughing, fatigue, vomiting, loose stools, diarrhoea, and other digestive symptoms. The tongue coating is generally greasy and has obvious dampness toxicity from the disease [21]. In the whole clinical process, attention must be paid to eliminating dampness, resolving turbidity, eliminating filth and removing toxicity, as well as the effects of blood stasis [19]. TCM has been used to treat epidemic diseases since ancient times and has accumulated abundant successful experience in the prevention and treatment of epidemic diseases for thousands of years [22][23][24]. The advantage of TCM is that, even if the cause of the disease is unknown, a set of corresponding prescriptions can be proposed based on the clinical symptoms under the guidance of the theory of syndrome differentiation and treatment, which can relieve clinical symptoms, shorten the course of the disease, and prevent deterioration due to the disease [25,26].

Clinical efficacy of TCM in the prevention and treatment of COVID-19
In the prevention and treatment of COVID-19, TCM is an irreplaceable main force [27]. Zhang et al. [28] reported that early treatment with TCM could reduce the inflammatory response, regulate immune function, promote the absorption of pulmonary inflammation, and reduce mortality in patients with severe COVID-19. The integration of traditional Chinese and Western medicine had a good effect on COVID-19, and it could improve the rate of alleviation of fever, coughing, expectoration, fatigue, chest tightness, and anorexia; shorten the duration of fever and coughing; shorten the hospital stay; reduce the severity of illness; promote the absorption of pulmonary inflammation and nucleic acid negative conversion; and improve the overall response rate and cure rate [29][30][31][32][33][34][35][36][37][38][39][40][41][42][43].

The relationship between the gut microbiota and TCM
The normal gut microbiota consists of bacteria, viruses, fungi, and archaea in numbers up to 10 13 -10 14 . On the one hand, under normal physiological conditions, the gut microbiota can interact with the host through complex dynamic network regulatory mechanisms, participate in regulating the host's material metabolism and immune defence, and protect the integrity of the intestinal mucosal barrier structure [117][118][119]. On the other hand, beneficial bacteria can produce antibacterial substances that are more beneficial to the body and resist the invasion of pathogens by competing for nutrients and occupying favourable sites [120]. When the homeostasis of the intestinal microecology is disrupted, beneficial bacteria decline, harmful bacteria increase, the intestinal barrier is damaged, and the function of the immune system is disrupted, thus triggering a variety of diseases and threatening human health. It has been reported that the gut microbiota is closely related to the inflammatory response induced by novel coronaviruses and could be highly correlated with individual susceptibility to COVID-19 [121]. Numerous studies have shown that respiratory virus infection of the host causes alteration of the gut microbiota, increasing the colonization of pathogenic bacteria and decreasing the number of probiotics. In turn, alteration of the gut microbiota can reduce the level of the host's antiviral immune response and aggravate the lung injury caused by viral infection. In addition, the gut microbiota can act directly on viruses through its structural components or metabolites or indirectly influence the body's immune response, ultimately affecting the outcome of viral infection [122]. Probiotics can enhance the antiviral ability of the body by regulating innate immunity, maintaining the integrity of the intestinal wall, and activating adaptive immune responses [123].
There is a close relationship between TCM and gut microbiota, as shown in Fig. 3. On the one hand, TCM can influence the composition and metabolism of the gut microbiota; on the other hand, the gut microbiota can metabolize Chinese herbal medicinal components [124]. A large number of studies have shown that TCM can significantly adjust the alteration of the gut microbiota, promote the growth of beneficial bacteria, inhibit the overproliferation of harmful bacteria, and balance the number of these probiotic and pathogenic bacteria, thus maintaining a healthy intestinal environment [125,126]. Most TCM is administered orally and is selectively metabolized in the intestine by intestinal secretory enzymes into active or absorbable components; TCM thus exerts a therapeutic effect. In addition, the gut microbiota will be affected by TCM while transforming it. The composition of the gut microbiota will be changed under the effects of TCM. On the one hand, TCM can alleviate disease-induced gut microbiota disorders by regulating the composition of the gut microbiota and then treating the disease; on the other hand, changes in the composition of the gut microbiota can also lead to changes in its metabolic ability, thus triggering gut microbiota-mediated interactions of Chinese herbal medicinal components. The gut microbiota in the body plays an important role in the pharmacological effects of oral TCM, and TCM helps to maintain the balance of the gut microbiota. The interaction between them is of great significance in preventing and treating diseases and promoting health [127].

The relationship between COVID-19 and gut microbiota
The initial symptoms of COVID-19 patients are mainly fever, coughing, and fatigue. With in-depth research on the disease, it has also not been uncommon to find gastrointestinal symptoms caused by SARS-CoV-2 infection in clinical practice [128], especially in critically ill patients with poor prognoses [9][10][11]. Available evidence suggests that SARS-CoV-2 can directly damage the digestive system, disrupt the intestinal microecological balance, and cause associated clinical manifestations [11,129,130]. Gastrointestinal symptoms are present in approximately 2-79.1% of patients [9,49,[131][132][133], and these symptoms can appear early in the onset of the disease, even earlier than fever and respiratory symptoms [134][135][136]. By studying 1099 patients with COVID-19, Nanshan Zhong's team found that the clinical manifestations of gastrointestinal symptoms, such as nausea, vomiting, and diarrhoea, could be 8.7%, and gastrointestinal symptoms were more common in severe patients, accounting for 12.7% [49]. Another study [9] found that 11.4% of COVID-19-infected people presented with gastrointestinal symptoms, such as abdominal pain, diarrhoea, nausea, and vomiting. Moreover, a study [137] confirmed that some COVID-19 patients had their first symptoms in the digestive tract, suggesting that gastrointestinal symptoms are another major manifestation of COVID-19 patients, in addition to respiratory symptoms. The Renmin Hospital of Wuhan University reported "atypical" cases in which patients with COVID-19 presented with gastrointestinal symptoms only, without fever or respiratory symptoms [138]. Wang et al. [132] found that the proportion of COVID-19 patients with gastrointestinal symptoms as initial symptoms accounted for approximately 10.12%, and the patients only had diarrhoea symptoms at the onset of the disease, followed by fever, dyspnoea, and other symptoms. Currently, SARS-CoV-2 has been identified and isolated from the stool specimens of COVID-19 patients [139,140], and disorders of the intestinal microecology have been detected in COVID-19 patients with gastrointestinal symptoms [130]. These findings suggest that SARS-CoV-2 might cause alterations in the gut microbiota along with pulmonary infection, which could be the main reason for the nausea, vomiting, diarrhoea, and other gastrointestinal symptoms in patients.
Alteration of the gut microbiota is closely related to the severity of COVID-19 patients [11,141]. A study found that COVID-19 patients with diarrhoea were in more serious conditions and had higher rates of receiving noninvasive ventilation and transferring to the intensive care unit and a higher case fatality rate than those without diarrhoea [11,142]. Another study that included 74 COVID-19 patients with gastrointestinal symptoms and 577 COVID-19 patients without gastrointestinal symptoms found that the proportion of COVID-19 patients with gastrointestinal symptoms who were in severe or critical condition was as high as 22.97% -higher than the 8.14% of patients without gastrointestinal symptoms [9]. In 138 COVID-19 study subjects, the incidence of gastrointestinal symptoms was higher in severely ill patients than in ordinary patients, and severely ill patients presented with abdominal pain, whereas mildly ill patients did not [143]. Numerous retrospective studies have indicated that more than half of COVID-19 patients are complicated with underlying diseases, such as diabetes mellitus, hypertension, and hyperlipidaemia, with poor tolerance and rapid disease progression, and they are prone to becoming severe and critical high-risk patients [49,131,132]. In an interview with a CCTV reporter, the academician Li Lanjuan mentioned that severe and critical COVID-19 patients mostly have gut microbiota disorders. Severe and critical COVID-19 patients are predominantly elderly patients complicated with underlying diseases [144]. The intestinal microecological stability and diversity of elderly individuals decrease, and infection with SARS-CoV-2 causes gut microbiota disorders, which tend to affect the absorption of nutrients and reduce mucosal barrier immunity; the risk of nosocomial infections is thus increased [145]. In summary, there are many links between the gut microbiota and COVID-19. On the one hand, COVID-19 patients have gut microbiota disorders and impaired barrier function; could have gastrointestinal symptoms, such as nausea, vomiting, and diarrhoea; and could also have gastrointestinal symptoms as the first manifestation. On the other hand, gut microbiota disorders can aggravate the condition of COVID-19 patients.

Mechanism of gut microbiota disorder induced by SARS-CoV-2
SARS-CoV-2 is the third coronavirus to emerge in humans over the past 20 years after severe acute respiratory syndrome coronavirus (SARS-CoV) and Middle East respiratory syndrome coronavirus (MERS-CoV) [146,147]. Studies have shown that the pathogenic process of SARS-CoV-2 infection is associated with the binding of angiotensin-converting enzyme 2 (ACE2) [129]. Current studies have shown that, similar to SARS-CoV, SARS-CoV-2 uses ACE2 with the help of transmembrane protease serine 2 (TMPRSS2) as a cellular receptor in the host cell [148][149][150]. There are many ACE2 proteins in alveolar epithelial cells. After SARS-CoV-2 enters alveolar epithelial cells, virus replication causes an immune response in the body, and excessive cytokine production eventually forms a cytokine storm, in turn triggering inflammation, which is an important cause of lung injury, acute respiratory distress syndrome (ARDS), and multiple organ failure or even death [25,151]. Notably, ACE2 is highly expressed not only in alveolar cells but also in oesophageal epithelial cells, gastrointestinal epithelial cells, and hepatobiliary duct cells [152][153][154]. The above evidence suggests that SARS-CoV-2 might not only infect the respiratory system but also directly infect the digestive system and cause related clinical manifestations [155]. The high expression of ACE2 in the intestine can regulate the composition of the gut microbiota [156]. ACE2 is necessary for the expression of the neutral amino acid transporter B0AT1 protein in intestinal epithelial cells, and it participates in the absorption of tryptophan [157]. When the level of ACE2 expression in the digestive tract is reduced, tryptophan is not efficiently absorbed, small intestinal mTOR pathway activity is reduced, and small intestinal antimicrobial peptide expression is inhibited, ultimately leading to alteration of the gut microbiota and an increase in endotoxin. These effects in turn trigger endotoxaemia and cytokine storms, leading to tissue damage and even ARDS and MODS [157][158][159]. Thus, the dysregulation of gut microbiota in COVID-19 might be related to decreased intestinal ACE2 expression caused by the binding of SARS-CoV-2 to the ACE2 receptor on the surface of intestinal epithelial cells, as shown in Fig. 4.

The relationship between cytokine storms and the gut microbiota
A cytokine storm, also known as a cytokine cascade or hypercytokinaemia, is essentially a more intense inflammatory response and an excessive immune response of the body to pathogenic microorganisms (such as bacteria and viruses) [160]. When pathogens invade the organism, a moderate immune response is activated, which can hinder viral replication and rapidly clear the pathogens. However, an excessive immune response leads to an imbalance in the immune regulatory network, and an excessive inflammatory response occurs in the body, prompting a large release of multiple inflammatory factors and cytokines into the body fluids in a short period and producing systemic inflammatory damage, resulting in lung injury, ARDS and multiple organ dysfunction syndromes (MODS) and even death [161]. Test reports of COVID-19 patients have shown that the absolute value of lymphocytes was reduced in most patients, suggesting that SARS-CoV-2 could mainly act on lymphocytes, especially T lymphocytes. The viral particles spread through the respiratory mucosa, infect other cells, induce a cytokine storm in vivo, produce a series of immune responses, and cause peripheral blood changes in immune cells, such as leukocytes and lymphocytes [131]. Clinical studies have shown that plasma levels of IL-2, IL-7, IL-10, granulocyte colony-stimulating factor (G-SCF), interferon-γ-inducible protein-10 (IP-10), monocyte chemoattractant protein-1 (MCP-1), macrophage inflammatory protein-1α (MIP-1α), and TNF-α in COVID-19 patients are elevated [131,151], and critically ill patients usually exhibit reduced peripheral blood T lymphocytes [132]. Abundant evidence has suggested that cytokine storms play an important role in the progression of COVID-19, especially in critically ill patients [11,162,163]. A recent COVID-19 clinical study [151] showed that a high level of inflammatory cytokine expression, the "cytokine storm", was found in critically ill patients infected with SARS-CoV-2, suggesting a possible correlation between cytokine storms and the severity of patients' infections. Cytokine storms are now considered a turning point in the progression of COVID-19 patients to severe and critical illnesses. Some patients with COVID-19 had mild disease in the early stages, with sudden aggravation in the later stages and eventual death from MODS, with exacerbation mainly due to a cytokine storm [164]. Reports have indicated that the cytokine storm in severe COVID-19 patients is closely associated with severe lung pathological injury and the occurrence and development of ARDS, and it is one of the main causes of death in patients [131,151,165].
Numerous studies in recent years have shown that intestinal microorganisms play important roles in regulating the immunity of the body and that changes in the gut microbiota are closely associated with local and systemic inflammation [166]. The microecological imbalance of the gut microbiota leads to the release of inflammatory mediators caused by intestinal endotoxin and gut microbiota displacement, directly or indirectly promoting the occurrence and development of the COVID-19 inflammatory storm. After SARS-CoV-2 infection, especially in elderly patients or those with underlying diseases, alteration of the gut microbiota increases the production of various toxins, including endotoxins, and impairs intestinal barrier function, causing a large number of toxins to enter the bloodstream. These toxins can directly damage lung structure and function, or they can aggravate the systemic and pulmonary inflammatory response by inducing inflammation and creating inflammatory factor storms, leading to severe disease. It was found that, among COVID-19 patients, the elderly and those with underlying diseases, such as diabetes, hypertensive disease, coronary heart disease, and extreme obesity, are susceptible to SARS-CoV-2, are prone to developing acute and critical illness, and have a higher risk of death [132,[167][168][169]. Previous studies [170,171] have shown that the richness, evenness, and diversity of the gut microbiota in this population were significantly reduced, and the dominant flora was significantly decreased, with an imbalance in intestinal microecology. Infection with SARS-CoV-2 further aggravates gut microbiota disorders, leading to disruption of the intestinal mucosal barrier, the release of inflammatory mediators caused by intestinal endotoxin and gut microbiota displacement, and the formation of an inflammatory factor storm to exacerbate systemic and pulmonary inflammatory responses. It is now believed that the cause of severe pneumonia is not the virus itself but the excessive immune response induced by infection. The unbalanced inflammatory factor system in the body is an important cause of pneumonia and ALI [172,173]. The above studies suggest that the gut microbiota is closely associated with the inflammatory storm in COVID-19 patients; that is, imbalance of the gut microbiota after SARS-CoV-2 infection, disruption of the intestinal mucosal barrier, and release of inflammatory mediators caused by intestinal endotoxin and gut microbiota displacement can directly or indirectly promote the occurrence and development of the COVID-19 "inflammatory storm".

Regulation of gut microbiota structure
Many studies have been reported on the direct regulation of the gut microbiota composition by TCM, including the regulation of the gut microbiota by a single TCM or a single extracted component, and on the forms of Chinese herbal medicinal ingredients that act. Guo et al. [174] found that extracts of Ginseng radix et rhizome rubra and Coicis semen could promote the growth of probiotics such as Lactobacillus and Bifidobacterium and inhibit the growth of pathogenic bacteria such as Escherichia, Staphylococcus, and Salmonella in vitro. Chang et al. [175] reported the effect of adding Ganoderma extract to the food of mice fed a high-fat diet and found that Ganoderma extract not only reduced the elevated ratio of the Firmicutes/Bacteroides phylum and the level of endotoxin-producing Proteobacteria induced by a high-fat diet but also maintained the integrity of the intestinal mucosal barrier and reduced the occurrence of endotoxaemia. The compatibility of essential oil from Atractylodis macrocephalae rhizoma and total ginsenosides reduced the relative abundance of opportunistic pathogens, such as Bacteroides, Anaerotruncus, and Desulfovibrio [176]. Dong et al. [177] found that Shenling Baizhu powder could effectively regulate antibiotic-induced gut microbiota imbalances and reduce endotoxin production by promoting the proliferation of probiotics, such as Lactobacillus and Bifidobacterium, in the intestinal tract. The single Chinese medicines Ginseng radix et rhizoma, berberine, Lonicerae japonicae flos, Rehmanniae radix praeparata, Artemisiae argyi folium, and emodin had proliferative effects on intestinal probiotics such as Lactobacillus in the gut microorganisms of different disease models [127]. HuoXiang Zhengqi [178] could support the growth of normal intestinal flora and promote the proliferation of beneficial bacteria (Bifidobacterium and Lactobacillus). Gegen Qinlian decoction could reverse the reduction in the richness of the gut microbiota, change its structure, and significantly increase the relative abundances of short-chain fatty acid (SCFA)-producing bacteria, including Akkermansia, Bacteroides, Clostridium, Ruminococcus, and Phascolarctobacterium [179]. Coptidis rhizoma extracts could improve intestinal microecology. Pathogens such as Enterobacter and Enterococcus were inhibited, and probiotics such as Lactobacilli and Bifidobacteria were notably promoted by Coptidis rhizoma extracts [180]. Fermented Yupingfeng polysaccharides exhibited greater beneficial effects in improving the gut microbiota, including augmenting flora diversity and the abundance of cellulolytic bacteria and reducing the abundance of Enterococcus spp and Streptococcus spp [181]. Based on 16S rDNA sequencing, Wu et al. [182] found that Qingfei Paidu decoction significantly modulated the composition of rat gut microbiota, upregulating the relative abundance of Romboutsia, Turicibacter, and Clos-tridium_sensu_stricto_1 and downregulating the relative abundance of norank_f_Lachnospiraceae, suggesting that the clinical efficacy of Qingfei Paidu decoction might be related to the regulation of gut microbiota composition.

Regulation of the intestinal mucosal barrier to prevent bacterial translocation
In addition to influencing the composition and number of gut microbiota constituents, TCM can indirectly affect the positioning of the flora by protecting the intestinal mucosal barrier. The normal intestinal mucosal barrier effectively prevents intestinal bacteria and endotoxin from crossing the intestinal mucosa, avoiding gut-derived infection. A variety of single TCMs, Chinese herbal medicinal ingredients, and TCM preparations have been found to have significant protective effects on the intestinal mucosal barrier. A modern pharmacological study showed that Rhei radix et rhizoma can effectively block the cascade response of inflammatory mediators, reduce the number of intestinal Escherichia coli bacteria in severely infected rats, inhibit the expression of Toll-like receptor 2 (TLR2) and TLR4 mRNA, block the intestinal inflammatory response, maintain the function of the intestinal mucosal immune barrier, and inhibit the translocation of bacteria in the early stage of septicaemia, with important therapeutic effects on acute lung injury [183]. Chang et al. [175] found that Ganoderma lucidum was able to protect the barrier function of the intestinal mucosa in mice, thus resisting the invasion of pathogenic microorganisms and preventing bacterial translocation. Fermented Yupingfeng polysaccharides (FYP) maintain intestinal barrier integrity and functionality by upregulating the mRNA levels of ZO-1, claudin, polyimmunoglobulin receptor, trefoil factor, and epidermal growth factor in the jejunum and ileum, achieving a protective effect on the mechanical and immune barriers of the intestinal mucosa [184]. Rhei radix et rhizoma can maintain intestinal mucosal immune barrier function by regulating the gut microbiota and suppressing intestinal inflammatory responses [183].

Regulating immunity
Recent studies have suggested that the causes of exacerbation and even death in viral infectious diseases such as COVID-19 are not only related to pulmonary viral infection but also closely related to immune dysfunction of the body [185,186]. Therefore, in the process of preventing and treating COVID-19, it is very important to improve the body's own immune ability and regulate the immune balance. In addition to the direct or indirect antiviral effects of TCM in the prevention and treatment of COVID-19, the greatest advantage of TCM lies in its excellent immunomodulatory effects and reduced toxicity and side effects [187]. TCM has played an important role in the prevention and treatment of COVID-19. Among its effects, the immunomodulatory effect of TCM cannot be ignored. There have been many reports on the direct regulation of immunity by TCM, not only in the form of a single extracted ingredient or single Chinese medicine but also in the form of a compound prescription of TCM.

Single extracted ingredients of Chinese medicine regulate immunity
Poria Cocos polysaccharides [188] could improve immunity by reducing the content of IL-10 and regulating the secretion of Th1/Th2. Radix glycyrrhizae polysaccharides [189] could regulate cellular immunity disorders in tumour-bearing mice by decreasing the proportion of Treg cells and increasing the spleen lymphocyte transformation ratio. Bordbar et al. [190] found that glycyrrhizin could enhance the expression of the surface differentiation antigens CD80 and CD86 and the    4. Mechanism of gut microbiota disorders induced by SARS-CoV-2 and the mechanism by which TCM regulates the gut microbiota to prevent and treat COVID-19. The binding of SARS-CoV-2 to the ACE2 receptor could reduce the expression of ACE2 and affect the absorption of tryptophan by small intestinal epithelial cells. It could also reduce the activity of the mTOR pathway in the small intestine and inhibit the expression of small intestinal antimicrobial peptides, leading to intestinal flora imbalance (a decrease in the number of beneficial bacteria and an increase in the number of harmful bacteria) and displacement, the destruction of the intestinal mucosal barrier, and an increase in intestinal endotoxins. The above process causes the excessive release of inflammatory mediators, the occurrence of endotoxaemia and cytokine storms, and tissue damage. In severe cases, ARDS and MODS can occur. However, TCM can help to suppress inflammatory storms and maintain the immune barrier function of the intestinal mucosa, thereby improving the body's immunity and preventing the deterioration of the condition because of TCM's ability to adjust the intestinal flora imbalance, promote the growth of beneficial bacteria, inhibit the excessive reproduction of harmful bacteria, and promote the production of SCFAs. major histocompatibility complex II (MHC II) of dendritic cells (DCs), improve the production of IL-12, enhance the proliferation of allogeneic T cells, increase the expression of IFN-γ and IL-10, and regulate the Th1-type immune response. Forsythia leaf polysaccharides [191] could significantly increase the thymus index, spleen index, macrophage phagocytosis, spleen lymphocyte proliferation, serum IL-2 and IL-4 levels, haemolysin content, and number of haemolytic plaques in immunosuppressed mice induced by cyclophosphamide. Baicalein [192] could play an immunomodulatory role by balancing CD4 + /CD8 + and regulating the balance of T lymphocytes and their Th1/Th2 and Th17/Treg subsets. Chrysophanol [193] had an immunomodulatory effect in immunocompromised mice, which could enhance the proliferation of T and B lymphocytes, increase the spleen index and thymus index, upregulate the levels of serum IL-2 and IL-4, and promote an increase in haemolysin levels and the production of antibody cells. Flos Lonicerae polysaccharides [194] could regulate the immune function of immunosuppressed mice induced by cyclophosphamide, effectively increasing the organ index of the mice, enhancing the activity of NK cells and the phagocytosis of macrophages, promoting the proliferation of spleen lymphocytes, and enhancing the immunomodulatory activity of the immunosuppressed mice. In addition, Jia et al. [195] found that Flos Lonicerae polysaccharides could promote IFN-γ secretion by regulating immune function, stimulating an increase in the immune organ index and enhancing immune function. Liu et al. [196] found that Astragalus polysaccharides could improve the phagocytosis rate of macrophages and the spleen and thymus indices of mice.

Single Chinese medicines regulate immunity
Astragali radix, Scutellariae radix, Isatidis radix, Pulsatillae radix, Polygoni cuspidati rhizoma et radix, Herba hedyotidis, and Andrographis herba could regulate the balance of anti-inflammatory cytokines and proinflammatory cytokines in the lungs of mice infected with H1N1 influenza virus and then improve immune dysfunction, truncate the storm of cytokines, reduce the inflammatory injury of lung tissue, and promote the repair of inflammatory lesions of lung tissue [197][198][199][200]. Astragali radix [197] reduced the mRNA expression of the proinflammatory cytokines TNF-α, IL-1, and IL-6, significantly increased the mRNA expression of the anti-inflammatory cytokines IL-10 and IFN-γ in the lungs of mice, and significantly inhibited and repaired immune-inflammatory injury in lung tissue in mice. Scutellariae radix [198] inhibited the protein and gene expression of TNF-α, IL-1 and IL-6 in the lungs, promoted the protein and gene expression of IL-10 and IFN-γ, reduced the immune-inflammatory injury of lung tissue, and promoted the repair of inflammatory lesions in lung tissue. Isatidis radix, Pulsatillae radix, Polygoni cuspidati rhizoma et tadix, and Herba hedyotidis can inhibit the expression of TNF-α, IL-1 and IL-6 and promote the expression of IL-10 and IFN-γ in the lung [199]. Andrographis herba [200] effectively regulated the concentrations of TNF-α, INF-γ and IL-10 in the peripheral blood of mice, increased the percentage of CD3 + T lymphocytes, regulated the CD4 + /CD8 + ratio, improved the cellular immune function of mice, and regulated the balance of proinflammatory and anti-inflammatory factors.

Chinese medicine compound prescriptions regulate immunity
Clinical studies have shown that Qingfei Paidu decoction can significantly improve the CD3 + and CD4 + cell counts of COVID-19 patients and enhance the body's immune function [107,201]. Toujie Quwen granules [80] could significantly increase the CD4 + count and the ratio of CD4 + /CD8 + in patients with COVID-19, promote the recovery of immune function and reduce the injury of cellular immunity. Lianhua Qingwen capsules [202] significantly inhibited the phosphorylation of p65 protein in the NF-κB pathway and significantly reduced the expression levels of TNF-α, IL-6, KC, MCP-1, IL-1β, and IP-10 in the lung tissue of H1N1-infected mice, suggesting that its immunomodulatory effect might be related to its inhibition of the NF-κB pathway. Jinhua Qinggan granules reduced the levels of serum CRP and IFN-γ in patients with influenza and improved immune function [203,204]. Xuebijing injection [205] reduced the levels of TNF-α and IL-6 in the lungs of mice infected with the influenza virus. In addition, clinical studies showed that, compared with human immunoglobulin alone, Xuebijing injection combined with human immunoglobulin could significantly shorten the antipyretic time, coughing, and asthma regression time and significantly reduce the levels of hs-CRP, IL-6, and TNF-α in patients with severe viral pneumonia [206]. Reduning injection [207,208] increased the level of IFN-γ in the lung tissue of virus-infected mice, decreased the levels of IL6 and TNF-α, decreased the levels of phosphorylated nuclear factor kappa B inhibitor protein (p-IκB) and NF-κB protein, decreased the expression of IL-1β mRNA and increased the gene expression of interferon-induced transmembrane protein 3 (IFITM3) and mitochondrial antiviral signal protein (MAVS). Tanreqing injection [209][210][211] could significantly improve the pathological injury of lung tissue in virus-infected mice, enhance the function of T and B lymphocytes, increase the content of IL-4 and IFN-γ in the lungs, downregulate the transcriptional activity of the transcription factor NF-κB, inhibit the content of TNF-α in the lungs, regulate the balance of Th1 and Th2 cells, reduce lung injury and corresponding inflammatory reactions, and enhance antiviral immune function. Buzhong Yiqi decoction [212] could significantly increase the serum CD3 + and CD4 + cell counts and the ratio of CD4 + /CD8 + in patients with recurrent respiratory tract infections and improve the immune function of patients. Maxing Shigan decoction [213] upregulated the body mass, spleen index, and thymus index of mice infected with influenza virus, reduced the levels of TNF-α, IL-1β, and IL-6 in the lungs, downregulated the expression of MyD88 and tumour necrosis factor receptor-associated protein 6 (TRAF6) mRNA and protein in lung tissue, increased the level of IL-2 in lung tissue and decreased the levels of IL-4 and TNF-α. Wang et al. [214] found that Sijunzi decoction could increase the levels of acetic acid, propionic acid, and butyric acid in the intestine; reduce the levels of the splenic index and serum inflammatory factors IL-2 and IFN-γ; and increase the ratio of CD+ 4/CD+ 8 in spleen-deficient mice, suggesting that Sijunzi decoction could increase the major SCFAs in the intestine and improve immunity in spleen-deficient rats.

Regulation of gut microbiota and its metabolites
The gut microbiota and their metabolites can affect the local or systemic immune function of the host. SCFAs are important products of dietary fibre fermentation by beneficial intestinal bacteria. The most common SCFAs are propionic acid, acetic acid, and butyric acid, with Bacteroidetes producing mainly propionic acid and acetate and Firmicutes producing mainly butyrate [215]. SCFAs not only have an important function in the metabolism of the body but also play a role in regulating inflammation and immune homeostasis [216]. Immunomodulatory effects associated with SCFAs include enhancing the barrier function of intestinal epithelial cells, reducing the induction of proinflammatory cytokines, stimulating the production of regulatory T cells, promoting intestinal mucosal homeostasis, preventing colonic inflammation, and enhancing the proinflammatory state of intestinal epithelial cells and leukocytes [217,218]. SCFAs can induce the production and differentiation of regulatory T cells, thus promoting the production of the anti-inflammatory cytokine IL-10, and they can strongly reduce the release of the proinflammatory chemokines CCL3, CCL4, CCL5, CXCL9, CXCL10, and CXCL11, thereby inhibiting leukocyte migration with a strong anti-inflammatory effect [219]. Studies have found that Firmicutes is an important source of butyrate, which is a major energy substrate for cells. The decrease in the ratio of Firmicutes to Bacteroidetes could directly affect the metabolism of dietary fibre by the gut microbiota, resulting in lower concentrations of SCFAs, and the decrease in Firmicutes could induce or exacerbate local inflammatory responses [220]. Prevotella, Ruminococcus, Coprococcus, Clostridium butyricum, and Lactobacillus all increase the content of SCFAs in the intestine, induce the production and differentiation of regulatory T cells in the intestine of mice, promote IL-10 production, and exert inflammatory inhibitory effects [221][222][223]. In contrast, overgrown Escherichia coli or Klebsiella pneumonia could promote the expression of chemokines such as CCL5 and CXCL10, thereby aggravating the degree of inflammatory injury [224]. A previous study showed that influenza A virus infection could cause structural disorders of the gut microbiota and immune dysfunction in mice, and Maxingshigan decoction had a protective effect against influenza virus-induced intestinal immune damage by upregulating the relative abundance of Firmicutes, Lactobacillus, and Coprococcus and downregulating the relative abundance of Proteobacteria and Escherichia, regulating the structure of the gut microbiota and affecting the production of chemokines [225]. Other studies have indicated that Chinese medicine could increase the production of SCFAs by modulating the gut microbiota to improve the barrier function of the intestine and inhibit the inflammatory response of the intestinal mucosa [179,226]. Zhang et al. [227] reported that Shenling Baizhu powder could increase the relative abundance of SCFA-producing bacteria (including Bifidobacterium and anaerobic bacteria) in the intestine of obese rats, reduce the levels of serum endotoxin, TNF-α, and IL-1β, and reduce systemic inflammation. TCM regulates the gut microbiota to prevent and treat COVID-19, as shown in Fig. 4.

Conclusions and prospects
In the prevention and control of this epidemic, early intervention, full participation, and classified treatment of TCM have made important and lasting contributions to the comprehensive control of the COVID-19 epidemic. Clinical studies have confirmed that TCM has certain advantages in improving the clinical symptoms of COVID-19 patients, inhibiting the storm of inflammatory factors, promoting the absorption of pulmonary inflammation, shortening the course of treatment, and promoting recovery. Currently, the focus and difficulties in the treatment of COVID-19 concern mainly the more severe and critical cases, which are also the main factors directly affecting mortality [27]. There is no definite and effective treatment for the more severe and critical COVID-19 patients in modern medicine, but the intervention of TCM or integrated traditional Chinese and Western medicine according to the clinical symptoms of the more severe and critical COVID-19 patients can help to reduce mortality and improve prognosis [22,25,27,228]. The gut microbiota in severe and critical COVID-19 patients is disordered, rendering it prone to bacterial infection and further aggravating the disease. Therefore, it is essential to maintain the balance of the gut microbiota.
There are many relationships between the gut microbiota and COVID-19. On the one hand, COVID-19 patients have gut microbiota disorders and intestinal barrier function damage; on the other hand, the gut microbiota can regulate the local and systemic immune system, becoming the turning point of the prognosis of the disease. Therefore, restoring and regulating the balance of gut microbiota are helpful for the rehabilitation of COVID-19 patients. In the Guideline on Diagnosis and Treatment of COVID-19 (trial 8th edition) [15], formulated by the National Health Commission of the People's Republic of China, it is mentioned that, for more severe and critical COVID-19 patients, microecological regulators should be selected in a timely and reasonable manner to restore and maintain intestinal homeostasis and prevent bacterial infection. In addition, the "four antibodies and two balances" suggested by the academician Lanjuan Li, that is, anti-virus, anti-shock, anti-hypoxaemia, anti-secondary infection, maintaining water-electrolyte acid-base balance and microecological balance, also clearly indicate the importance of using microecological agents to maintain the balance of intestinal bacteria in improving patients' immunity and preventing secondary infection. TCM plays a positive role in promoting the prognoses of patients with coronavirus infection or COVID-19 by maintaining the dynamic balance between the type and number of the gut microbiota and improving body immunity, but its mechanism remains to be further elucidated. From the perspective of TCM, research on maintaining the gut microbiota balance and correcting gut microbiota imbalances and finding new targets and new ideas for the treatment of COVID-19, whether for the treatment of the sick or the health care of the undiseased, are of great significance.

CRediT authorship contribution statement
Shuping Wang was responsible for the entire manuscript; Zhihua Yang, Yangxi Liu, Lin Wang and Shanshan Lin conceived, designed, wrote and revised the manuscript; Zhihua Yang and Yangxi Liu have contributed equally to this work and shared first authorship; Xiangdong Dai, Haifeng Yan contributed to draw diagram; Zhao Ge, Qiuan Ren, Hui Wang, Feng Zhu revised the manuscript and discussed interpretation. All authors have read and approved the final submission. .