COVID-19 was first reported in December, 2019 in Wuhan, China, and rapidly spread across the globe ¹. It has affected more than 6.4 million people and has led to the death of over 370 thousand as of June 2, 2020 (www.who.org). Severely affected patients present fever, dry cough, dyspnea, and fatigue, which are commonly associated with the development of pneumonia and acute respiratory distress syndrome (ARDS) ². Advanced age, ischemic and congestive heart disease, hypertension, diabetes, and chronic obstructive pulmonary disease (COPD) are the most important independent predictors of death ^2,3. As with other infectious diseases affecting the respiratory tract, asthma has been cited as a potential risk factor for severe COVID-19^4-6; however, no previous study has addressed this specific question.

Here, we systematically reviewed all papers published on COVID-19 since its emergence in December 2019 to May 18, 2020, looking into the description of asthma as a premorbid condition and its putative association with severe progression of the disease.

Two authors, NFM and CPJ, independently identified cross-sectional and longitudinal studies published before May 18, 2020, that reported on the prevalence of asthma as a premorbid condition of severe COVID-19 by systematically searching PubMed-NCBI database. We restricted the search for PubMed-NCBI because COVID-19 is a new medical condition and, currently, PubMed-NCBI covers more than 90% of MEDLINE providing a widely accessible biomedical resource ⁷. For database searches, language of the article was restricted to English. Search terms included the following: COVID-19 (COVID, COVID 19) ornCov or novel coronavirus or Sars-Cov-2 in the title and clinical characteristics or asthma anywhere in the text. Three authors, EM, EPA, and LAV, resolved eventual discrepancies by discussion and adjudication.

We found 458 articles that met the initial inclusion search criteria (Supplementary Figure 1). All articles were assessed by authors and 290 were excluded (Supplementary Table 1) due to one or more of the following criteria: editorials; metanalyses; systematic reviews; commentaries; letters to the Editor; no description of patient’s clinical characteristics or premorbid conditions; and main text in a language other than English.

The remaining 150 articles were included in the study. Supplementary Table 2 depicts the details of all articles analyzed. As a whole, the articles described the clinical aspects of 36,072 COVID-19 patients. One hundred and seven studies mentioned the existence of other respiratory premorbidities except for asthma. Asthma was mentioned as a premorbid condition in only eighteen studies (Table 1). There was a total of 8,690 patients included in the studies mentioning asthma, and 655 patients were previously diagnosed with asthma. In most of the studies describing other respiratory illnesses, COPD was the leading diagnosis.

Based on the current medical records, we conclude that 7.5% of patients included in articles describing the clinical characteristics of COVID-19 patients and citing asthma were previously diagnosed with asthma. If all studies providing any clinical description of COVID-19 comorbidities are taken into consideration, asthma was present in only 1.8% of patients. These numbers are far less than expected considering the prevalence of asthma in the world. According to the World Asthma Report (http://www.globalasthmareport.org), there were as many as 339 million people living with asthma in the world in 2018, which corresponds to 4.4% of the world’s population.

In conclusion, asthma does not seem to be an important premorbid condition in COVID-19 patients; or, conversely, it could be a protective factor, as previously proposed ⁸. The findings herein reported could be an epidemiological truth that should be further explored in mechanistic studies or could be due to the fact that researchers are not properly investigating and describing the premorbidities in COVID-19 patients. Whatever the reasons, the medical community should be aware of the implications of missing the diagnosis of a potentially severe respiratory disease such as asthma that could worsen the prognosis of COVID-19 patients.

Acknowledgements. NFM was supported by The São Paulo Research Foundation (grant: 2016/17810-3), and CPJ was supported by Coordination for the Improvement of Higher Education Personnel (CAPES) grant: 1744875 and 88882.434715/2019-01. EM, LAV and EPA are supported by grants from São Paulo Research Foundation (grants: 2013/07607-8 and 2020/) and Brazilian National Council of Scientific and Technological Development (CNPq).

Ethics. The study does not require ethical approval because the systematic review is based on published research and the original data are anonymous.