## 4 Discussion  
Our case-control study found a prevalence of asthma of 8.9% in ED patients with COVID-19, similar to that reported in the general adult population and in most studies of patients hospitalized for COVID-19. Furthermore, in our study, asthma was not a risk factor for hospitalization or critical COVID-19. The prevalence of asthma worldwide in 2019 ranged from 5.4 to 9.8%, based on different definitions of asthma [2]. As shown by a census carried out by Italian general practitioners, the national prevalence was 9.1%, similar to the 8.8% observed in the Lazio region [21]. As showed by several studies published in the last two years, the prevalence of asthma in adults with COVID-19 was extremely variable. It ranged from 0.32% to 25.8%, with a pooled prevalence between 8.08% and 9.38%, based on studies included in a recent meta-analysis [22; 23; 24]. Based on the available limited data from Italy, the prevalence of asthma in adults with COVID-19 was lower than expected. Two retrospective cohort studies on patients hospitalized for COVID-19 in Northern Italy showed a prevalence of asthma of 2.1% and 3.8%, respectively [12; 25]. In a brief report by our group on a retrospective cohort of 734 newly diagnosed COVID-19 patients admitted to the ED of our hospital, asthma prevalence was 2.5% [18]. To the best of our knowledge, in addition to the results produced by our group, only one other study has been published regarding the relationship between asthma and COVID-19 in the emergency department setting. It was a large retrospective cohort (4558 individuals) enrolled in United States (New York City, Bronx), reporting a high prevalence (20.9%) of asthma [19]. The extreme variability in asthma prevalence observed in these studies of COVID-19 patients is difficult to explain. It could depend on several factors, including geographical and racial differences, pandemic wave, setting, sample size, and other study-related methodological factors. The same variables may have also influenced the conflicting results of the studies assessing whether asthma was a risk factor for a worse COVID-19 outcome [10; 11; 12; 13; 14; 15; 16; 17]. This is demonstrated by several meta-analysis published in recent months, choosing to include or not very heterogeneous studies, which sometimes presented asthma as a protective or neutral factor [22; 23; 24; 26] and sometimes, as a risk factor for worse outcome [27; 28; 29]. However, especially when looking at more recent and higher quality studies, asthma does not appear as a risk factor for a worse outcome of COVID-19. As previously mentioned, most of the published studies enrolled hospitalized patients, thus creating a bias related to the loss of information regarding what happens before hospitalization. The emergency department can be a good middle ground between what has happened outside the hospital, concerning which it is very difficult to obtain sufficiently detailed information (see the large insurance or government datasets), and what happens inside the hospital wards. As mentioned, our study is the only one, along with that of [PERSON] et al., which enrolled patients admitted to the ED [19]. The proportion of asthmatics hospitalized for COVID-19 (78.8%) was extremely similar to that observed in our case-control study (84.3%). The outcomes explored were different, but mortality was similar in patients with asthma, without other significant comorbidities, and controls without asthma and with a comparable comorbidity burden. On the other hand, our findings that asthma is not a risk factor for critical COVID-19 were consistent with more recent prospective and retrospective studies on hospitalized patients [10; 11; 12; 13; 14]. Some pathophysiology recall could help explain why a chronic respiratory disease associated with morbidity and mortality, especially during exacerbation triggered by infectious agents, does not represent a risk factor for SARS-CoV-2 infection and for severe COVID-19.