1. Introduction

The winter of 2019 marked the initial spread of the COVID-19 outbreak, caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) [1]. Coronavirus is categorized as an RNA virus within the subfamily coronaviridae [2]. The novel zoonotic outbreak has been traced to Wuhan, China, starting in December of 2019. According to the Centers for Disease Control and Prevention (CDC), the COVID-19 pandemic has resulted in over 35 million cases, and over 611 thousand deaths in the United States (US) alone, as of August 2021 [3]. Individual and environmental factors play a role in an individual’s susceptibility to COVID-19 [4]. The CDC has reported an increased risk of sickness and death among racial and ethnic minorities, disabled individuals, and older adults, as 95% of deaths are among individuals over the age of 45 [5]. The spread of COVID-19 occurs both via cross-species and human-to-human interaction [2]. Transmission of COVID-19 occurs via respiratory droplets and aerosols containing the virus, along with direct contact transmission ^[6][7][6,7]. These forms of transmission include but are not limited to close contact with individuals sneezing or coughing, contact with host mucosal membranes of eye, nose, mouth, and medical procedures such as bronchoscopy that generate aerosols [8]. With an incubation period of 5–14 days, COVID-19 is seen to be spread by both infected asymptomatic and symptomatic individuals [9]. Symptoms of COVID-19 include fever, cough, dyspnea, fatigue, shortness of breath, muscle aches, among other manifestations ^[10][11][10,11]. Viral binding to the host target cell results in interleukin-6 (IL-6) production and the activation of the nuclear factor kappa B (NF-κB) pathway, resulting in a proinflammatory state characterized by an increase in macrophage and cytokine concentrations. The presenting cytokine storm and immune dysregulation of COVID-19 may develop acute respiratory distress syndrome, organ failure, coagulation, and more [9]. This cytokine storm response can lead to T-cell exhaustion, seen often in chronic infectious states. Patients with COVID-19 have been found to have decreased levels of CD4+ T-lymphocytes (< 200 cells/μL), which increases susceptibility for fungal infection development ^[12][13][12,13]. Given that CD4+ T-lymphocytes play a role in the effective immune response to presenting pathogens, they indicate a patient’s immunologic status and functioning ^[14][15][14,15]. Research suggests that viral respiratory diseases, such as COVID-19 may predispose an individual to other fungal, bacterial, and viral coinfections and superinfections ^[16][17][16,17]. Superinfection, occurring subsequently, and coinfection, occurring concomitantly, cause greater difficulty and complication in diagnosis due to an overlap of symptoms and consequently complicate the treatment of COVID-19 [Table 1]. Such multi-infectious states often rtesult in a worse outcome than either infection alone ^[18][19][18,19]. Fungal infections, for instance, often have similar symptoms to COVID-19, such as cough, shortness of breath, and fever, making it difficult to distinguish between the two diseased states ^[20][21][20,21]. A summary of such symptoms has been provided in Table 1. Common fungal infections seen associated with COVID-19 infection include Aspergillosis, Candidiasis, Cryptococcosis, and Mucormycosis [21]. These infections are caused by fungi Aspergillus genera, Candida Auris, Cryptococcus neoformans, and fungi of Mucorales order, respectively. Fungi cause a variety of diseases in both immunocompetent and immunocompromised individuals. Fungal infections can develop as primary or secondary to other diseases, with modes of infection and risk varying with the pathogenic fungi that ultimately result in activation of the immune system [22]. A multi-infected state may function to increase systemic inflammation and consequently prolong recovery, leading to increased use of treatment methods, need for intensive care, and risk of death [23].

Aspergillosis infections typically occur in immunocompromised individuals. Risk factors for invasive aspergillosis include corticosteroid therapy, viral infections, and lymphopenia, amongst others ^[29][76]. Invasive pulmonary aspergillosis (IPA) has previously been observed in patients with influenza and has been shown to cause more severe disease and increased mortality when compared to patients who had influenza without invasive pulmonary aspergillosis ^[30][77]. There are immunopathological similarities between influenza and COVID-19, such as cytokine storm syndrome, tissue damage, lymphopenia, and impaired coagulation ^[31][78]. As the COVID-19 pandemic continues to spread, many patients are at risk of coinfection with aspergillosis, which may be difficult to diagnose and worsen patient outcomes. In one study, clinicians evaluated the incidence of IPA in 108 patients with severe COVID-19 and found that 27.7% of these patients also developed COVID-19 associated pulmonary aspergillosis (CAPA) (Table 2).

They also found that patients diagnosed with probable CAPA had a significantly higher 30-day mortality rate than patients with COVID-19 who did not meet the criteria for Aspergillosis ^[32][79].

COVID-19 may result in damage to the respiratory epithelium, allowing aspergillosis to invade tissue ^[33][80]. Treatment with steroids may be a risk factor for patients with CAPA as one study found that three out of a total of five patients with CAPA were treated with steroids, and all three of them died, while the other two who did not receive steroid treatment remained alive ^[34][81]. An evaluation of COVID-19 intensive care patients found a strong association between the use of high dose systemic corticosteroids and aspergillus coinfection ^[35][82]. A study observing patients with COVID-19 who were admitted to an intensive care unit found an association with receiving Azithromycin for 3 or more days and being diagnosed with probable IPA. Researchers proposed that as Azithromycin has immunomodulatory properties, its use may be a risk factor for developing IPA in patients with COVID-19 ^[36][83].

Moreover, IL-6 is a proinflammatory cytokine with significantly elevated levels in severe COVID-19 patients and has also been found to play a role in protection against Aspergillus ^[48][84]. Tocilizumab is approved for use in patients with COVID-19 as it is a potent IL-6 inhibitor. While Tocilizumab may be used to treat COVID-19, it may promote a secondary coinfection, such as aspergillosis, as it functions to reduce serum IL-6 levels ^[37][85]. While these therapies have been shown to contribute to coinfections, there have been investigations into other therapies. A 2021 paper evaluated the use of thymosin alpha 1 (Tα1), all-trans-retinoic acid (ATRA), and lactoferrin against opportunistic fungal infection ^[49][86]. Tα1 demonstrated a protective effect against Aspergillus fumigatus in an experimental murine model of bone marrow transplantation. Tα1 increased the Th1 immune response against Aspergillus fumigatus ^[49][86]. ATRA inhibits in vitro growth of Aspergillus fumigatus via enhancing macrophage phagocytosis. ATRA also showed a synergistic effect with antifungal drugs such as amphotericin B and Posaconazole ^[49][86]. Lactoferrin demonstrated antifungal activity against Aspergillus fumigatus via a possible mechanism of iron sequestration and inducing direct cell membrane damage ^[49][86]. The use of steroids, Azithromycin, and Tocilizumab in patients with severe COVID-19 should be monitored closely as such therapies can lead to an Aspergillus coinfection. More research is needed to investigate the clinical use of natural immunomodulators further.

Candidiasis is a fungal infection caused by the yeast Candida. Candida commonly lives on the skin and mucosal surfaces, including the oropharynx, intestinal lining, and urinary tract. Candida are typically commensal fungal species; however, if certain conditions are met, they can become invasive and cause candidiasis ^[50][87]. Hospitals across several countries have observed COVID-19 associated candidiasis (CAC). One study found that Candida sp. was the most prevalent COVID-19 associated fungal coinfection making up 18.8% of such cases [19]. A review found that the prevalence of CAC ranged from 0.7% to 23.5% across several countries ^[38][88]. A CAC outbreak in ICUs at a hospital in Mexico resulted in a mortality of 83.3% among patients with candidemia ^[39][89]. Understanding the factors associated with driving CAC can protect COVID-19 patients from candida coinfections, in addition to allowing clinicians to promptly diagnose and treat CAC.

Researchers from Iran found that COVID-19 patients with oropharyngeal candidiasis (OPC) had at least one of the following risk factors: lymphocytopenia, ICU admission, mechanical ventilation, corticosteroid use, broad-spectrum antibiotic use, or an immunocompromised condition (Table 2). Of these risk factors, broad-spectrum antibiotic use was the most common as it was present in 92.5% of OPC patients ^[51][90]. As antibiotic use may disrupt the balance between oral bacterial and yeast populations, it can create an environment that permits candida overgrowth and infection ^[52][91]. Clinicians in Turkey observed that COVID-19 patients who were receiving broad-spectrum antibiotics were at an increased risk for developing candidemia ^[40][92]. In COVID-19 treatment guidelines, the World Health Organization recommends against the use of broad-spectrum antibiotics unless there is a clinical suspicion of bacterial infection ^[53][93]. A study done in Italy found that three critically ill patients with COVID-19 developed candidemia after treatment with Tocilizumab, an IL-6 inhibitor ^[41][94]. A trial assessing the susceptibility of candidiasis in IL-6 deficient mice found that IL-6 deficient mice had increased mortality and higher candida fungal loads when compared to control mice ^[54][95].

A case-level analysis found that 25.5% of patients with candidemia were also positive for COVID-19 and that ICU admission, mechanical ventilation, catheter placement, steroid and immunosuppressant use were 1.3 times more common in these patients when compared to patients who had candidemia but were COVID-19 negative ^[42][96]. Arastehfar et al. also found that 74.5% of patients with COVID-19 associated candidemia infections had undergone central venous catheterization ^[38][88]. Catheter placement has been implicated in the introduction and proliferation of microorganisms such as Candida, which pose therapeutic problems as they can form biofilms ^[55][97]. Candida colonization is common in mechanically ventilated patients, as long-term ventilation is associated with a significant increase in respiratory and urinary tract candida populations ^[56][57][98,99]. As previously mentioned, Gaziano et al. demonstrated the potential of various natural immunomodulators (Tα1, ATRA, and Lactoferrin) against opportunistic fungal coinfections ^[49][86]. In vivo and in vitro experimental studies showed the remarkable antifungal activity of Tα1 against systemic Candida albicans infection. Tα1 potentiates polymorphonuclear cell-induced intracellular killing of the fungus ^[49][86]. In vitro, ATRA can be used as a fungistatic drug by inhibiting the growth of Candida albicans. Lactoferrin, through its ability to sequester iron, showed strong antifungal activity ^[49][86]. Clinicians must be aware of risk factors for nosocomial candidiasis coinfections in patients with COVID-19, particularly when such patients are treated with broad-spectrum antibiotics, corticosteroids, Tocilizumab, especially in a background of catheter placement or mechanical ventilation. More research is needed to further investigate the clinical use of natural immunomodulators.

Cryptococcosis is a fungal infection caused by cryptococcus species and can be fatal in immunocompromised individuals. It is one of the more prevalent infections in patients with HIV and AIDS ^[58][100]. Previous data found that 81% of patients with cryptococcosis developed sepsis and that the 30-day fatality rate of such cases was 37% ^[59][101]. To date, there have only been a handful of case reports on COVID-19 associated cryptococcosis infections (Table 2). While it appears to be a rare occurrence, investigating potential risk factors and causes for COVID-19 associated cryptococcosis is important. The infection can quickly become fatal if not identified and treated appropriately, especially in immunocompromised individuals with HIV/AIDS.

In one case report, a patient with a history of kidney transplant and liver cirrhosis who was COVID-19 positive later developed a cryptococcus coinfection and did not survive. The researchers suggest that CD4+ T-cell depletion caused by COVID-19 may have been a key driver for cryptococcosis in this case; however, they could not draw a definitive conclusion as cryptococcal infections have also been associated with both solid organ transplant and liver cirrhosis patients independent of COVID-19 ^[43][102]. Another case report found that a COVID-19 positive patient treated with Tocilizumab and corticosteroids went on to develop cryptococcemia and died within 10 days ^[44][103]. Previous research has shown an association between high levels of IL-6 and resistance to cryptococcal infection ^[60][104]. The most recent case report presented a patient with COVID-19 who was treated with dexamethasone and developed severe cryptococcal meningitis ^[45][105]. The authors suggest that the impact of steroids on T-cell function should be further investigated, as T-cell depletion has been shown to be a driving factor for cryptococcal meningitis.

Mucormycosis is an infection caused by fungi in the order of Mucorales, most frequently by the Rhizopus spp., Lichtheimia spp. and Mucor spp., which account for nearly 75% of all cases ^[46][106]. The most common route of infection is through inhalation of spores that lead to pulmonary infection, typically in immunocompromised individuals. Cutaneous and soft-tissue manifestations may also be common, and in diabetic populations, rhino-orbital mucormycosis is commonly presented. In a meta-analysis, diabetes mellitus was the most common comorbidity contributing to the development of rhino-orbital mucormycosis in 340 of 851 (40%) patients with an odds ratio of 2.49 (95% CI 1.77–3.54) compared to the next possible factor of having hematological malignancies with an odds ratio of 0.76 (0.44–1.26) ^[46][61][106,107]. Risk factors for mucormycosis include diabetic ketoacidosis, corticosteroid treatment, organ/bone marrow transplantation, neutropenia, trauma/burns, and elevated levels of free iron ^[62][63][30,108].

Mucormycosis, popularly known as black fungal infection, is an emerging disease, with the occurrence in the general population cited as 0.005 to 1.7 per million ^[60][104]. However, in India, the prevalence of mucormycosis is close to 0.14 cases per 1000 population, nearly 80 times its prevalence in developed countries ^[64][109]. The surge COVID-19 cases in India had been associated with increased reports of invasive mucormycosis post-COVID-19 and are continuously being reported to be rising ^[47][110]. While many treatment options have been evaluated for COVID-19, glucocorticoids have been shown to improve survival but, on the other hand, can lead to secondary fungal infections (Table 2). The combination of SARS-CoV-2, steroid overuse, and uncontrolled diabetes mellitus has contributed to a significant increase in the incidence of invasive mucormycosis ^[65][68]. Another contributing virulence factor that plays an important role in the pathogenesis of mucormycosis is its ability to uptake free unbound iron from the host. Hyper-ferritinemic states such as diabetic ketoacidosis, iron-chelator treatment in dialysis, or severe COVID-19 can further predispose an individual to mucormycosis ^{[66][67][68][69]}[54,63,64,111].

Treatment for mucormycosis involves surgical debridement whenever possible, in addition to systemic antifungal therapy with liposomal Amphotericin B as the choice of drug ^[61][107]. However, despite surgery and antifungal treatment, the overall mortality rate for mucormycosis remains over 50% and approaches 100% in patients with disseminated disease and neutropenia ^[70][112]. Thus, COVID-19 treated patients who are diabetic and were administered corticosteroids for controlling the severity of infection may be more susceptible to mucormycosis infections with poor prognosis, further complicating the pandemic scenario by leading to more fatalities.