Our study, which analysed all nationwide AECOPD hospitalisations from 2012 to 2017 using 3 separate analyses, resulted in three main findings: First, AECOPD during influenza seasons did not have worse outcomes than outside influenza seasons, with shorter LOS and ICU admission but similar mortality and only slightly increased risk of rehospitalisation. Second, outcome was better if influenza was detected during hospitalisation for AECOPD than if not, with lower mortality and risk of rehospitalisation, similar LOS and only slightly increased risk of ICU admission. Third, in patients with AECOPD and pneumonia, influenza diagnosis was associated with increased risk of ICU admission but no significant difference in any other outcome. Complications in form of aspergillosis were rare and not higher in any of our sub-groups. Overall, these data confirm that the clinical outcome of AECOPD in Switzerland was not affected by likely or confirmed influenza diagnosis thereby supporting the safety of the current management including corticosteroids.
Clinicians in Swiss hospitals usually adhere to local guidelines or practices which are typically based on recommendations of the Centers for Disease Control and Prevention or European Centre for Disease Prevention and Control to determine when and in what patients influenza testing should be done, but there are no countrywide directives with general validity. Therefore, in common clinical practice, testing for influenza virus infection outside the influenza epidemic period in patients without major suspicion for influenza is rarely performed. As a result, AECOPD with influenza may be considerably underdiagnosed – especially outside influenza epidemic period - and subsequently erroneously diagnosed and coded as AECOPD without influenza infection. Because of their simplicity, relatively low costs and fast results, during influenza epidemic rapid influenza diagnostic tests (RIDTs) are still a widely used method for influenza testing [10][23]. As RIDTs show a high specificity, but a lower sensitivity compared to real-time polymerase chain reaction (RT-PCR) testing or the newer rapid molecular assays, many cases of influenza infections as triggers for AECOPD may have been missed as they were classified as (false) negative results[24]. For these reasons, we performed separate analyses: The first analysis comparing AECOPD during vs. outside the influenza season likely included most influenza episodes but was not very specific as there still are many other infectious and non-infectious causes for exacerbation even during influenza periods, particularly for patients vaccinated against influenza. This analysis showed discordant results with no difference in in-hospital mortality, a significantly shorter LOS but slightly higher risk of rehospitalisation for patients admitted during influenza season. An increased awareness for respiratory symptoms of COPD patients during influenza season might have led to lower threshold to hospitalise them in case of suspicion for influenza-caused exacerbation and therefore quicker initiation of appropriate treatment including antivirals with faster clinical response[25], even though we did not have access to medication data. This might also explain the significantly higher rate of rehospitalisations for AECOPD and the lower need for ICU admission during influenza periods. The hypothesis of lower hospitalisation-threshold and therefore better outcomes during influenza season mirrors the results of a study on asthmatic and non-asthmatic patients with influenza, where earlier hospital admission and the early use of corticosteroids – both associated with asthmatics – were found as an explanation for better outcomes compared to hospitalised non-asthmatics[19]. Earlier hospitalisation together with generally higher hospital occupancy during winter and spring resulted in greater pressure to discharge patients, and might have been at least partially responsible for the observed shorter mean LOS during influenza periods.
In contrast, comparing outcomes for AECOPD in patients with versus without influenza diagnosis in the second analysis would be most specific for influenza but miss many episodes likely caused by influenza[26]. There was a slightly increased risk of ICU admission but no significant difference in terms of LOS, while in-hospital mortality and rehospitalisation rates were significantly lower in patients with influenza. This supports the notion that corticosteroids, which we assume were also given in AECOPD with influenza coinfection and in the slightly more frequently occurring ICU settings of influenza positive patients, did not worsen the outcome of those exacerbations, even though the use of steroids had shown a negative impact as adjunctive treatment in influenza alone[13][27]. Consistent with this, we recently showed that COPD patients had less pneumonia-related complications, which possibly may be due to their use of inhaled corticosteroids[28]. These results confirm previous data from Korea and Hong Kong, that severity and outcomes were similar between AECOPD with or without viral detection [29, 30]. In contrast, older data suggested that there was a larger drop in peak flow and a longer recovery time in patients with viral AECOPD [25]. This might be confounded by the observation that viral exacerbations were more frequent in patients with higher GOLD stages [31]. A recent Canadian study showed that among 4755 patients with COPD hospitalised during influenza seasons, those with influenza diagnosis had significantly higher rates of mechanical ventilation, ICU admission and mortality than influenza test-negative patients.[32]
Finally, the special situation of patients who had an exacerbation of COPD with pneumonia - for which there is a German guideline recommendation [18] - was analysed by comparing outcomes between those with influenza diagnosis and those without. Here we also found an increased risk of ICU admission but no significant differences in any other outcome, which supports the current recommendation for corticosteroids if an AECOPD occurs simultaneously with an influenza infection. Of note is the low number of cases (637 hospitalisations in five years) in the influenza test-positive group and the high percentage of ICU admissions in both groups. No significant differences between groups in terms of complications such as aspergillosis or empyema were detected in our nationwide study which is also reassuring.
In general, our study did not find consistent evidence of a worse outcome in patients with AECOPD and any of our surrogates of influenza infection (epidemic period, laboratory-confirmed diagnosis) compared to patients without those surrogates. Of note, in all three analyses, there was one outcome (either risk of re-hospitalisation or risk of ICU admission) which was worse in the presumed influenza group, however other parameters and most importantly mortality were either not different or in favour of the presumed influenza group. If we assume in the absence of medication data in this database, that Swiss patients with AECOPD are generally - and in accordance with the 2020 GOLD report and current recommendations [33] - treated with corticosteroids, irrespective of a diagnosis of influenza or the respiratory season, our data supports the safety of corticosteroids in the majority of patients with AECOPD including those due to influenza. Even though influenza virus replication is increased in the presence of corticosteroids [34] and was shown in a recent meta-analysis to result in higher mortality and more nosocomial infections in influenza-associated severe pneumonia and acute-respiratory distress syndrome [35], these effects were not present in the population of patients with COPD in our study. As possible explanation we hypothesize that chronically obstructed lungs react differently and the major clinical determinants in this situation are rather the obstruction and inflammation[36], which are reduced with corticosteroids, rather than the viral cytopathic effect of influenza, which would be exacerbated by corticosteroids. However, the exact reasons for these observed differences between patients with and without COPD are not well understood.
Limitations
While the retrospective nationwide study design allowed to analyse a large number of hospitalisations, it has limitations as all retrospective studies including detection bias.
By using coding data, our study is dependent on coding quality. Coding errors such as carrying over an exacerbation from a former hospitalisation in patients with stable COPD might have resulted in erroneous inclusions or exclusions of hospitalisations. Nevertheless, to code and bill a diagnosis, a criterion of related work-up or clinical evidence has to be present, and as coding of hospitalisation data became revenue-relevant for the stationary healthcare service providers in Switzerland in 2012, it is performed by professional coders ensuring high coding quality. Still, as a result of being a fairly young domain, coding may still be in a process of consolidation and have undergone slight changes throughout the period of our study.
Another weakness of our study was the lack of baseline characteristics (e.g. exact age or smoking status) in the dataset.
The main weakness is the lack of medication data in the database. With this, we can only make assumptions on how frequently corticosteroids are used in AECOPD in general and in particular when influenza is diagnosed or suspected. Nevertheless, as for hospitalised patients with AECOPD the use of systemic steroids is standard of care in Switzerland and we are confident that most patients received them.
The main strength of this study is the size of the dataset which includes data from all hospitalisations in Switzerland and provided more than eight million entries of hospitalisations from 2012 to 2017 thereby avoiding selection bias. This made it possible – in contrast to other study designs, which extrapolate from a random sample to the entire population– to accurately assess the total amount of hospitalisation cases in Switzerland. This resulted in more than 30’000 cases for the smallest analysis and almost 70’000 for the bigger ones, what lies beyond most clinical trials and supports the robustness of our observations.