Respiratory infections caused by the influenza virus can develop severe pneumonia and acute respiratory failure requiring Intensive Care Unit (ICU) admission. One of the possible complications of influenza pneumonia is the presence of community-acquired respiratory co-infection (CARC).1 Our group reported an incidence of coinfection close to 17%, and it was observed an independent association of CARC and ICU mortality.1

In recent years, the incidence of Influenza-Associated Pulmonary Aspergillosis (IAPA) has increased, and currently the reported incidence rates range from 9% to 23%.2–5 The appearance of IAPA worsens the prognosis in mechanically ventilated patients, with a mortality rate reported between 50% and 60%.2,3,6 However, the incidence of IAPA in critically ill patients and the risks factors related to the development of this entity vary among currently available reports, possibly due to the populations considered, the different definitions used, and the diverse active microbiological search strategies carried out.

Knowing the incidence of Aspergillus spp. isolation (AI) in critically ill patients would help to better characterize the affected type of population and therefore to identify the need to initiate early empirical antifungal treatment. In this context, the aims of our study were to determine the incidence and impact on ICU mortality of AI during the first 24h of admission in a large population of critically ill patients admitted due to proven severe influenza infection.

This is a secondary analysis of an observational and prospective cohort study. Data were obtained from a voluntary registry created by the Spanish Society of Critical Intensive Medicine and Coronary Units (SEMICYUC, Spanish acronym), in which 184 Spanish ICUs participated between June 2009 and June 2019. The study was approved by the Ethics Committee of the Joan XXIII University Hospital (IRB # 11809) and by each of the participating centers. Informed consent was not obtained from the patients given the observational nature of the study, and the anonymity of the data was guaranteed.

The main finding of our study is that the incidence of AI in respiratory sample of critically ill patients with severe influenza pneumonia is low in Spain, and it was closely associated with crude ICU mortality. Another interesting finding is that COPD did not appear as a AI associated factor, while hematological disease or other causes of immunosuppression were highly related with AI development.

To our knowledge, this is the first study to asses the incidence and impact of AI in patients with severe influenza pneumonia within the first 24h of ICU admission. Other studies have evaluated the Aspergillus incidence in an ICU evolution period. This difference makes it difficult to compare between studies. However, given the impact of our results, we believe it is important to highlight some considerations of clinical relevancy.

In our study, AI incidence is notably lower regarding the incidence of IAPA reported by other authors in other countries.3–5 The association between pulmonary aspergillosis (PA) and influenza infection or other respiratory viruses has been described previously.11–14 This association can respond to various factors. Viral infections directly injure the epithelium of the bronchial mucosa,11,12 facilitating the invasion by fungi present in the respiratory tract. Furthermore, influenza infection alters cytokines levels at both pulmonary and systemic levels, and the function of pulmonary macrophages and T-lymphocytes, producing a certain degree of immunodeficiency that favors Aspergillus spp. infection.13,14

The association between PA and viral infection is frequent in immunosuppressed patients. In a cohort of 219 patients receiving an allogeneic transplant,15 viral respiratory tract infection was shown to be a risk factor for the development of PA (HR: 4.3, 95% CI: 2–9.4). García-Vidal et al.4 reported an 8.8% incidence of IAPA in 57 immunosuppressed patients admitted to hospital due to influenza A(H1N1)pdm09 virus infection during the 2009 pandemic. The same author16 reported an association between influenza virus infection (HR: 2.5, 95% CI: 1.1–5.8) and parainfluenza (HR: 2.6, 95% CI: 1.6–4.3) with invasive infection by filamentous fungi. However, in critically ill patients IAPA incidence and risk factors are uncertain and highly variable.17–19 The low incidence of AI in our population (1.1%) differs from that published in other series. Wauters et al.5 observed a 23% incidence of PA in a cohort of 40 patients with influenza A(H1N1)pdm09 infection. The Dutch/Belgian mycosis study group has recently published3 an incidence of 19% of IAPA in patients with influenza A(H1N1) virus infection, clearly higher than the 5% observed in the control group of patients admitted for bacterial community-acquired pneumonia. Finally, this group also found a higher incidence of PA within the influenza cohort group of immunocompromised patients versus non-immunocompromised patients (32% vs 14%). These differences with respect to our results may be related to the characteristics of the studied populations, especially regarding the frequency of immunosuppressed patients, as well as the implemented diagnostic methods and the observation period. The determination of galactomannan in BAL, used in some studies20 for aspergillosis diagnosis, is a rarely used technique (<20%) in invasive mechanically ventilated patients with community-acquired pneumonia 21. The importance of immunosuppression in patients with hematological disease as a risk to AI has been confirmed by our automatic CHAID analysis; this relation was described by Vanderbeke et al.22 in a recent review. In this regard, the GLIMP study,23 a multicentric, international, observational point-prevalence study for patients hospitalized due to community-acquired pneumonia, reported that Aspergillusfumigatus as the causative pathogen was found more frequently in immunocompromised patients (8 [1.3%] vs 10 [0.4%]; p<0.01) and that AIDS (OR: 15.10, 95% CI: 6.36–35.88) and hematological cancer (OR: 4.65, 95% CI: 1.85–11.69) were independent risk factors for A.fumigatus.

Male sex, asthma, nosocomial-acquired influenza infection, and immunosuppression were the variables associated with the risk of presenting AI in our cohort of critically ill patients. The relationship between pulmonary disease (e.g., asthma and COPD) and PA has been previously studied. It has been proposed that either the chronic inflammatory disease itself or its management, including chronic corticosteroids treatment and broad-spectrum antibiotic therapy, could be the factors which explain this association.24,25 Notably, COPD was correlated with a lower risk of AI in our study. In a similar line, Schauwvlieghe et al.3 found that COPD was not an independent risk factor for IAPA (OR: 1.10, 95% CI: 0.53–2.26, p=0.80). Several studies describe COPD as a risk factor for aspergillosis, especially in patients receiving chronic corticosteroid treatment.19,26–29 In our population, the proportion of patients with COPD was similar in the group with and without AI, and within COPD patients, the use of corticosteroids was similar in patients with and without aspergillosis (1.8% vs 1.5% respectively, p=0.8, data not shown).

The association observed between nosocomial-acquired influenza and AI could be explained by the increased risk of hospitalized patients to acquire influenza as an expression of a subclinical condition of impaired immunity or immuno-paralysis associated with the serious processes that led to hospitalization, something that facilitates the development of opportunistic infections.30 In this regard, patients with AI were admitted in the ICU with a higher median of hospital days previous admission than patients without AI, but these differences were no significant in the multivariate analysis.

Finally, AI patients presented three times greater ICU-mortality than those in whom AI was not present (59% vs 21%). This high mortality coincides with the one published by Schauwvlieghe et al.3 with a mortality of 45% in patients with aspergillosis compared to 20% in the rest of the cohort patients, and with the study published by Vanderbeke et al.22 who observed a mortality of 57% in patients with aspergillosis and influenza infection.

The main strength of our study is that it is a homogeneous, multicenter cohort with a numerous number of critically ill patients with confirmed influenza infection, which allows the results to be generalizable. Nonetheless, we must recognize several limitations. First, it was a secondary analysis of an observational study, and although patients have been included consecutively, we cannot rule out possible biases in reporting the cases; yet, the high number of patients and the rigorous statistical analysis performed minimize these risks and make the results generalizable. Another major limitation is the fact that we cannot confirm proven PA diagnosis in most of our patients because we only have microbiological isolation and imaging tests; furthermore, we do not have information about in how many of our patients a bronchoscopy had been performed and diagnosis was made by BAL. In this way, there is the possibility that “colonized” patients may have been included. However, the low incidence of AI and high associated mortality make this scenario unlikely. Moreover, these are real-world data, because this is what clinician do during the current clinical practice, which makes our results very important for patient care and clinical practice. Another limitation could be related to the low number of patients with AI included. Despite this, the observed differences in mortality have achieved significance with a post-hoc calculated statistical power of 89%. Finally, it is possible that the low incidence observed is due to the lack of an active search investigation through the determination of galactomannan in BAL. Although we cannot rule out this possibility, this technique is not available in all hospitals in Spain.

Due to the low incidence of AI in our population of critically ill patients with influenza infection, the extensive use of empirical antifungal treatment in all patients may not be justified. In moderate-high risk patients, the active search for Aspergillus spp. should be systematically implemented.

This research received no external funding. This study was supported by the Spanish Intensive Care Society (SEMICYUC) and Ricardo Barri Casanovas Foundation (Alejandro Rodríguez). The study sponsors have no role in the study design, data collection, data analysis, data interpretation, or writing of the report.

L.C., X.D., I.M.-L., M.B., L.F.R. and A.R. designed the study, design of statistical analyses, and data analysis. All authors except G.Q., J.P, L.F.R. and J.J.G. contributed to the patient's data for the general database. L.C., X.D., I,M-L., M.B., F.G.-B., C.S., E.P., E.D., S.T., J.P., G.Q., J.M-C., L.F.R., J.S.-V., M.B-S., R.Z., J.J.G.., A.R. contributed to critical examination of the paper for important intellectual content and approval of the final manuscript. All authors have read and agreed to the published version of the manuscript.

The authors declare that they have no conflict of interest.