In this study, the investigators will examine the extent to which having suffered coronavirus disease 2019 (COVID19) impacts one's sensibility to hypoxia by means of the 'Richalet test'. The aim of the study is to formulate recommendations for advice in altitude mountain medicine for patients having suffered COVID19. To determine any eventual changes in response to hypoxia, performances by participants having suffered COVID-19 and participants having stayed free of COVID-19 will be both compared intra-individually with previous performances (pre-COVID-19 pandemic) and between both groups of subjects. The investigators hypothesize that patients having suffered COVID19 might perform differently on the cardiopulmonary exercise test compared to before the illness. Based on recent research on COVID19 pathophysiology and -patient follow-up, it might be expected that COVID19 alters the response to hypoxia, thus influencing one's acclimatization capabilities at high altitude, albeit reversibly and/or temporarily.

Different alterations of response to hypoxia could be observed. The virus causing COVID19, the "severe acute respiratory syndrome coronavirus 2" (SARS-CoV-2), has the potential to significantly damage the nervous system and to affect cardiorespiratory functions. If SARS-CoV-2 does, similarly to MERS and SARS, induce cardiorespiratory and neurological dysfunction, then COVID19 patients may have impaired hypoxia response after infection and perform worse on the 'Richalet test' in comparison to before the illness.

Conversely, reports of high prevalence of dyspnea in patients up to 3 months after SARS-CoV-2 infection, might indicate infection-induced degenerative changes in the carotid bodies, which might lead to sensibilization of the peripheral chemoreceptors to impaired oxygenation. Possibly similar to the impact of aging and smoking on the cardiorespiratory response to hypoxia, this phenomenon of sensibilization could entail an increased hypoxic response in patients having suffered COVID-19. Accordingly, patients might perform better on the 'Richalet test' post-COVID-19 than they did before.

In this study, the focus will be on the portion of COVID-19 survivors which contemplates to travel to and (temporarily) reside in high altitude regions (>2500m). It is well known that as altitude increases, the barometric pressure falls, proportionally paralleled by a decreasing partial pressure of oxygen. At high altitude, this leads to a condition which is referred to as a hypobaric hypoxic environment. The dramatic drop in partial pressure of inspired oxygen and subsequent reduction in arterial partial pressure of oxygen implicates significant adjustments for the human body to survive at high altitude. In order to study the possible long-term effects of COVID-19 on oxygen transport physiology in these patients, the investigators will focus on the eventual cardiorespiratory and neurological consequences of SARS-CoV-2 infection and relate them to the physiological demands placed on the body by hypoxia at high altitude. This case-control study will be conducted by means of the 'Richalet Test', a hypoxia cardiorespiratory exercise test which has been validated for mountain medicine consultations to detect patients at risk of developing an inadequate response to hypoxia at high altitude. From the patients who came for consultation between 2015 and 2020, participants have been recruited by mail for the Coronaltitude study. All included participants, divided into those having suffered COVID-19 (COVID+ group) and those having stayed free of COVID-19 (control group), will retake an altitude mountain consultation. Results will be compared in between and within both groups with previous performances to determine if the response to hypoxia has changed in people having undergone COVID-19.

Inclusion Criteria:

• Subject having suffered COVID19 in the 12 months before inclusion, attested by a positive PCR, positive serology test or positive chest CT scan. (COVID+ group).
• Subject having stayed clear of COVID19 (COVID-/control group).
• Subject having been well informed and having provided written informed consent before participation.
• Subject covered by social security of some sort.
• Subject with an oxygen saturation of SpO2 > 95% in ambient air on day of Richalet test performance.
• Subject presenting with no symptoms of COVID19 (anymore) on the day of the experiment.
• Subject having already performed the Richalet hypoxia exercise test as part of the altitude mountain consultation in the years 2015 to 2019 in any of the 13 hospital centers participating at the study.

Exclusion Criteria:

• Subject with a history of respiratory, cardiovascular, neuromuscular, metabolic or renal pathologies.
• Subject with a history of psychiatric or behavioral disorder.
• Subject covered by L1121-5 to L1121-8 sections of the Public Health regulations (Code de la Santé Publique).
• Subject under guardian- or curatorship.
• Subject without social insurance.
• Subjet under the age of 18.
• Subject refusing to participate in the study.
• Subject diagnosed with an infection by a pathogen other than SARS-CoV-2.

All subjects, whether partaking in the COVID+ group or control group, got the same intervention. As such, the 'intervention' rather is the having undergone COVID19 or not. Involving sanitary reasons surrounding the COVID pandemic, a lack of communication concerning the COVID diagnosis between and to any party would have led to unnecessary health endangerment of concerned parties.