Immunocompromised patients are among the high risk groups to be infected with the novel SARS-CoV-2 virus. The immunocompromised, more frequently tend to be admitted in the COVID-19 ICUs, more frequently end up with mechanical ventilation and have higher mortality rates (15). In a New York City study of 90 SOT patients, the mortality rate of COVID-19 was reported to be 18%, significantly higher than the normal population (16). Efforts to maintain the intact function of the transplanted organ and concurrent reduction of the immune suppression dose to prevent severe viral disease, makes the management of COVID-19 in SOT patients remarkably challenging.
Due to immunosuppressive therapy, infections are already a frequent hazard in management of SOT patients. Bacteria are the most common pathogens causing infections after SOT, particularly in the early post-transplantation period (17, 18). Multiple previous hospitalizations, invasive interventions and prior antibiotic use makes the SOT patients a susceptible population to colonize nosocomial and antibiotic resistant pathogens including MRSA, VRE and resistant gram negative bacilli (19). On the other hand, community acquired infections may cause severe diseases in SOT patients, for instance bacterial or fungal superinfections exacerbate the common viral respiratory diseases in this vulnerable patient population (20).
Co-infections in general COVID-19 patients are not a frequent event, thus routine administration of antimicrobial agents is not recommended (21). Bacterial and fungal infections are reported to co-exist with SARS-CoV-2 to be 8% among 806 patients integrated in a meta-analysis of the available worldwide studies (22). In another meta-analysis of 3834 COVID-19 patients, the prevalence of co-infections was calculated to be 7%, in accordance with the previous study. In the study, Mycoplasma pneumonia, Pseudomonas aeruginosa and Haemophilus influenza were the most common organisms to cause secondary bacterial infections (23).The superinfections were reported to occur as high as 44% in critically ill ICU patients (24). Up to this date, few comprehensive studies of co-infections in COVID-19 SOT patients have been conducted. In this study we collected the clinical data of the 66 hospitalized SOT patients diagnosed with COVID-19 by positive SARS-CoV-2 PCR. The prevalence of bacterial or fungal infections was 21.2% among our patient population which was as predicted, higher than non-SOT COVID-19 patients. In a French report of COVID-19 course in kidney transplant recipients, 23.5% of the patients had a secondary bacterial infection (25). Patients with superinfections are at a higher risk of requiring ICU care and mechanical ventilation (26). The incidence of bacterial co-infections in newly admitted COVID-19 patients is relatively low (27). However, according to our study results, as ICU stay prolongs, the risk for co-infections increases. Furthermore, the need for mechanical ventilation increases the risk to acquire hospital-associated secondary pneumonia (28). In the current analysis, prior hospitalization and high dose corticosteroid dose were associated with increased risk of co-infection in SOT patients. In a previous study, older age was stated to be associated with more prevalent co-infections (29). Nori et, al. have reported that 100% of the patients with MDR co-infections had received prior antibiotics (30). Antimicrobial-resistant pathogens were frequent among our positive cultures, warning for an attentive antibiotic stewardship and avoiding empiric antibiotic prescription. Due to prior hospitalizations as a consequence of transplantation complications and receiving broad-spectrum antimicrobial agents, the high prevalence of MDR pathogens in SOT patients is a logical assumption (31, 32). Of the MDR pathogens, VRE is of special concern as VRE infections contribute to various complications and 2-fold increased mortality of the liver transplantation patients (33). The prevalence of VRE is differently reported worldwide and has high prevalence in some areas (31). In our study, the most common isolated bacteria in COVID-19 SOT patients were VRE species. The majority of VRE infections occur in pre-colonized patients (34). In a previous evaluation of pre-transplant patients in our center 9% were colonized with VRE (35). Furthermore, the majority of the isolated Candida species were of the non-albicans type (80%). The emergence of non-albicans Candida species is a global phenomenon, as the use of azole antifungals is being frequently applied (36). In the field of SOT, azoles are regularly prescribed as an anti-fungal prophylaxis. Therefore, an increasing trend in infections with non-albicans Candida rather than Candida alibicans is being observed (34).
Altogether, the most common type of secondary infection in COVID-19 patients is pneumonia, with blood stream and urinary tract infections standing at the next positions. (26). In the current study, urinary tract and sputum were the most common sites of pathogen isolation in the SOT patients, similar to the general COVID-19 patients. Ventilator-associated pneumonia was the most common co-infection in a surveillance of 52 SOT patients by Roberts et, al (37). Superinfections should carefully be paid attention to, as they can be the terminal event resulting in death (38) . Secondary bacterial infections were detected in 50% of the COVID-19 patients who had died, according to Zhou et, al (8). Martins-Filho et al, have reported a 2.5 fold mortality rate of COVID-19 in presence of co-infections (39). In our study, the existence of bacterial and fungal co-infections contributed to a significantly high mortality rate (35.7%), while a small proportion of the non-co-infected group passed away (0.019%).
Differentiating COVID-19 progression and a secondary bacterial infection is challenging, as the inflammatory markers might be elevated in absence of co-infections. In our patient population, the co-infected group had a dramatically higher mean CRP level. CRP is usually elevated in the COVID-19 patients as a result of ongoing inflammation. In an extensive Chinese review, 60.7% of the patients had high blood CRP level (40). Furthermore, a higher CRP level is associated with a more severe COVID-19 pneumonia and is an early predictive index of disease severity (41, 42). Procalcitonin (PCT), the precursor of the thyroid hormone calcitonin, is produced by extra-thyroidal tissues during inflammatory response to bacterial components (43) . In non-bacterial inflammations, PCT is either in normal range or slightly elevated, which makes it a more desirable biomarker rather than CRP to predict bacterial co-infection (44). One limitation of our study was, due to its retrospective nature, the scarce measurement and limited data of PCT levels in patients. Another limitation of our study was the limited number of patients, since it was a single center experience. Further studies of COVID-19 and co-infections in SOT patients are encouraged.