The current coronavirus pandemic is already one of the biggest global crises and poses an extreme challenge not only for the healthcare systems but is also a real test for social solidarity. Although the infection with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) leads in the majority of the cases to light, moderate or even no symptoms it can also have severe to fatal consequences. Current knowledge of this disease shows that not only the lungs but also the cardiovascular system, the skeletal muscle, the central as well as the peripheral nervous system, the blood and the immune system, the liver, the kidneys and further body functions might be affected [1–6].

According to current knowledge, athletes are not a risk group predisposed for a severe course of COVID-19 disease. However, this fact does not prevent athletes from infection with SARS-CoV-2 that could be associated with moderate or even stronger acute symptoms [7]. In this context, more severe courses have been described in, prior to infection, fit and healthy athletes. Myocardial involvement in terms of myocarditis, as described as a possible cardiovascular sequela in COVID-19 [2], requires special attention in athletes not only with respect to return into competitive sport (return to play, RTP). In a small case sample of 26 US college athletes, who were tested positive for SARS-CoV-2 and presented with none or only mild symptoms, four males met criteria for myocarditis in cardiac magnetic resonance imaging (cMRI) [8]. In addition, a study including 789 North American professional athletes, who were tested positive for SARS-CoV-2, reported abnormal findings in 30 cases during the cardiac screening, whereas five of these were diagnosed with inflammatory heart disease (myocarditis or pericarditis) by additionally performed cMRI [9]. In a group of 90 young adult competitive athletes, cardiac consequences of SARS-CoV-2 has been described in 3.3% of the cases with one athlete with myopericarditis and two cases with pericarditis [10]. In a larger cohort of collegiate athletes, the prevalence of clinical myocarditis as assessed on a symptom-based diagnostic algorithm was 0.3%. In the same cohort, additional MRI revealed that 2.3% of the athletes met criteria for clinical and subclinical myocarditis [11]. Additional data showed a slightly lower prevalence (1.5%) in young competitive athletes after COVID-19 as detected by primary cMRI [12]. However, the true prevalence as well as the clinical relevance of these findings are not completely clear at present [13].

In a meta-analysis, exercise-induced dyspnea has been described as one of the leading symptoms, persisting longer than 2 weeks after COVID-19 diagnosis [14]. Initial data in the actual pandemic reveal a relevant percentage of COVID-19 patients with persisting respiratory symptoms and an impairment of pulmonary function longer than 100 days after initial diagnosis [15]. With respect to athletic performance, it must be taken into account that even minor restrictive changes might limit maximum ventilation and/or disrupt respiratory economy. In addition, a disturbance of gas exchange as a result of a diffusion disturbance is likely to restrict exercise tolerance and performance. Furthermore, there is preliminary evidence of a compromised oxygen transport to the consuming tissues, which occurs at the erythrocyte level [16].

In addition to cardiac, pulmonary and other sequelae, COVID-19 has been shown to be at risk for persistent symptoms and delayed recovery, actually termed as “long-COVID-syndrome” [17]. A meta-analysis, which assessed more than 50 long-term effects extending the acute phase of COVID-19 showed that 80% of COVID-19 patients suffered from at least one symptom persisting more than 2 weeks after initial diagnosis [14]. Persisting symptoms with highest frequencies were fatigue (58%), headache (44%), attention disorder (27%), hair loss (25%) and dyspnoea (24%). It is assumed that at least 10% of the COVID-19 survivors develop a long-COVID-syndrome, which may underestimate the real prevalence [18]. To date it remains speculative, how many of the athletes are affected and to what extent “long-COVID-syndrome” is present in physical active subjects and in particular competitive athletes, exhibiting only mild or no symptoms during the acute phase of the infection.

The complex and at this time point incompletely understood clinical picture of COVID-19 disease leads to the question of how reintegration into sport can be achieved with an acceptable risk after overcoming the acute phase of the COVID-19 infection [19]. For this purpose, a number of recommendations have been published [2, 20–22], which summarize primarily expert consensus statements. Thus, for the development of more evidence-based RTP concepts there is a need for extensive research in this field [23]. Because of the current uncertainty of long-term course of COVID-19 disease, the same is true with respect to the necessity of long-term follow-ups in affected athletes [24].

From today’s perspective, it is unclear whether and to what extent a mild or even asymptomatic COVID-19 infection poses an acute health risk and a more chronic impairment of athletic performance in competitive athletes. The identification and envisaged quantification of such impairments are an important decision-making basis for the management of the pandemic risk of this infection in sports and for athletes’ reintegration and return into competitive sports. In this context, it is important to note that even a slight decrease in physical performance, not perceivable in daily living, may compromise competitive success in elite athletes. Nevertheless, to delineate subtle perturbations of COVID-19 disease on exercise performance and its underlying pathophysiology may also have importance for the non-athletic population. With respect to the prevalence of COVID-19 in competitive athletes, reliable epidemiological data is still lacking. Since asymptomatic or oligosymptomatic courses are presumably common in younger people [12], a high number of unreported cases of passed infections has to be expected, also and maybe particularly among competitive athletes. Nevertheless, it is unclear whether the prevalence of COVID-19 infection in competitive athletes differs from that of the general population in the same age-group. Moreover, it is unclear, whether there may be differences within the athlete population, depending on the type of sports and performance level, the grade of intensive physical contacts and the extent of sports-related travel activities. As described, the COVID-19 pandemic represents one of the greatest challenges also for competitive sports and, in particular, its sports medicine monitoring. In our opinion, this challenge can only be addressed in a multicentric approach. The objectives of our project are to investigate open questions about the type and frequency of potential health risks and remaining symptoms, to assess consequences of this disease on athletic performance and to determine the seroprevalence of COVID-19 diseases in the athletic population. In this context, close networking of the sports medicine centers in Germany is also useful against the background that although the infection rates exhibit intermediate drops and vaccination rates increase, the occurrence of virus mutations [25] and an unclear duration of immunity after vaccination clearly reflect that the pandemic is still in progress.

As the COVID-19 pandemic still affects competitive sports all over the world, knowledge of its impact on health of the athletes is still sparse, which emphasizes the urgent need for the present cohort study. If we are successful in recruiting more than 1,000 subjects in C1 and 2,500 or more in C2, CoSmo-S will represent the largest study in competitive athletes on this field so far in Germany. Reaching this sample sizes will allow us to perform substantial subgroup analyses that might provide valuable information on potential sports associated factors for risk of COVID-19 infection or mid-to long-term consequences for competitive athletes. Moreover, in these well characterized two cohorts, additional in-depth research can be conducted to address more specific questions such as effects of COVID-19 on T cell function. This should help to reveal the extent and magnitude of potential detrimental effect of SARS-CoV-2 infection even in athletes and physical active people. Additional subgroup analyses may be performed with respect to the started vaccination program against SARS-CoV-2 in the athlete population.

One key strength of our cohort study is that pre-study examination results are available from the majority of the squad athletes in C2 and in a lower frequency also in C1. This allows not only a more accurate detection of the effects of the pandemic on the ergometric performance of the athletes but also a more subtle assessment of pulmonary function, e.g., by comparing flow-volume curves in the spirometry with individual findings from the pre-pandemic time.

As we measure objective physical fitness by standardized ergometry, we can link these fitness data with initial and probably ongoing symptoms in the post-COVID cohort (C1). In turn, we hope to delineate predictors (e.g., symptom characteristics) for the individual risk to develop a drop in physical function and/or prolonged fatigue, which may also serve as a model for the general population. Moreover, negative effects of a COVID-19 disease on physical fitness and exercise tolerance are at risk to have detrimental effects on public health at all. As the recruited athletes individually perform a broad range of different types, volumes and intensities of training, it might be possible to assess the impact of different physical activity patterns on Long-COVID and to delineate underlying factors contributing to fatigue and other sequelaes. It is planned to compare our data with those obtained in other cohorts recruiting subjects from the general population.

Nevertheless, some limitations of the study merit consideration: In C1, athletes present themselves voluntarily to our outpatient clinics for post COVID-19 diagnostics and RTP examination, baring the risk for self-selection bias. It cannot be excluded that those athletes mainly present themselves in our clinics who have noticed acute symptoms or persistent complaints. In this context, it seems to be helpful that we can additionally calculate the percentage of symptom-free, but SARS-CoV-2 positive tested squad athletes also from C2. We hope that the results obtained in this cohort study will allow us formulate reliable recommendations regarding RTP on an evidence-based level.