Since the beginning of 2020, the outbreak of the novel coronavirus disease (COVID-19: coronavirus disease 2019), caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), has developed into a worldwide pandemic, resulting in 199,466,211 confirmed cases and 4,244,211 deaths worldwide (World Health Organization/WHO; August 04, 2021) (1). According to the Robert Koch Institute (RKI), as of August 05, 2021, there were 3,780,985 confirmed COVID-19-cases and 91,730 persons have died from the disease in Germany (2).

Due to the rapid spread of COVID-19 and the lack of information on virus transmission, especially at the beginning of the pandemic, and the lack of hygiene concepts, healthcare professionals (e.g., physicians or nurses) have been the most vulnerable to COVID-19. According to the daily status report of the RKI for the coronavirus disease on June 25, 2021, 91,043 healthcare professionals have been reported as testing positive for COVID-19, 35,799 of whom work in hospitals and 2,030 of whom work in rehabilitation facilities. On that day, 3% of all infected healthcare professionals required hospitalization, no cases of death were registered and 90,800 patients were classified as recovered. A systematic review by Goméz-Ochoa et al. (3) showed that healthcare professionals show a higher prevalence of SARS-CoV-2 (18% of the 97 studies reviewed from 2020) compared to the general population. According to the RKI statements, it is presumable that the number of affected healthcare professionals will increase. According to the German Social Insurance Code (§9 SGB VII) (4) and the Ordinance on Occupational Diseases [Appendix 1 to the “Berufskrankheiten-Verordnung (BKV)”], COVID-19 can be recognized as an occupational disease 3101 when the insured person suffers from infections due to an activity warranting insurance protection. Therefore, to receive coverage, infection with SARS-CoV-2 must occur during the performance of the insured person's profession, e.g., in the field of (1) healthcare (e.g., hospitals, physicians' offices, pharmacies, nursing services), (2) social services (e.g., facilities for the care of children, youth, families, the elderly, and disabled persons, or persons suffering from mental illnesses as additional care), (3) laboratory work, or (4) activities with an increased risk of infection compared to (1) through (3). On the other hand, persons must be notably exposed to the danger of infection, suffer from at least marginal disease symptoms, and demonstrate intensive contact to the origin of the infection (“index person”) (5). According to the German Social Accident Insurance (DGUV, Department Statistics), 92,175 cases recognized with occupational disease 3101 were confirmed in Germany as of July 20, 2021. Furthermore, COVID-19 can be recognized as a work-related accident if infection with SARS-CoV-2 occurs because of the insured person's occupation without meeting the requirements for an occupational disease 3101. This requires being able to trace the infection back to the respective insured activity (e.g., employment, university/school, exercise of certain honorary positions). In this context, there must be intense and direct contact with an infectious person (“index person”) and the disease must manifest within 2 weeks of the last contact. Work-related accidents are terminated, externally influenced incidents that result in damage to health or death. Data from the DGUV (Department Statistics, July 20, 2021) confirmed 7,741 recognized cases of COVID-19 as work-related accidents. In the following, COVID-19 as an occupational disease or work-related accident will be referred to as Occupationally Acquired COVID-19 or OAC.

In general, the course of COVID-19 is diverse, non-specific, and variable, both in terms of presenting symptoms and severity of disease. According to the WHO, disease severity can be categorized into the following stages: mild (patients with symptoms but without evidence of viral pneumonia or hypoxia), moderate (with clinical signs of pneumonia), severe (with severe pneumonia and indication for oxygen therapy), and critical (e.g., with acute respiratory distress syndrome, thromboembolism, and/or multi-organ failure). Depending on disease progression, patients may require admission to a hospital or even an intensive care unit (ICU) with invasive mechanical ventilation. To date, the following predictors of a severe or critical course have been identified: Age (>50 years), male sex, dyspnea and persistence of fever, severe lymphocytopenia, and an increase of biomarkers (e.g., high sensitivity of C-reactive protein, proinflammatory cytokines), or underlying comorbidities (e.g., hypertension, heart diseases, diabetes, obesity, respiratory diseases, tumors, physical inactivity) (6–9). COVID-19 is also recognized as a multi-organ disease with the following extrapulmonary manifestations (10): neurological symptoms and diseases (e.g., dizziness, neuropsychiatric symptoms) or encephalopathy (11), cardiovascular symptoms and disease (e.g., myocardial damage, heart failure) (12, 13), renal, hepatic, and endocrine diseases (10), gastrointestinal symptoms, dermatological symptoms (10, 14).

According to the NICE (National Institute for Health and Care Excellence) guideline (15), there are clinical definitions for the initial illness and long-COVID at different times. Acute COVID-19 is defined as signs and symptoms of the disease for up to 4 weeks. The guideline refers to ongoing symptomatic COVID-19 when signs and symptoms last for a period of 4 up to 12 weeks. Post-COVID-19 syndrome is characterized as signs and symptoms for more than 12 weeks that cannot be explained by an alternative diagnosis and developed after an infection consistent with COVID-19. The term long COVID-19 refers to continuing signs and symptoms following acute COVID and includes ongoing symptomatic COVID-19 and post-COVID syndrome. At the beginning of the COVID-19 pandemic, little was known about long-lasting symptoms following acute COVID-19. The current state of research now indicates various physical, cognitive, and psychological symptoms in affected patients which persist over weeks or months and depend on disease severity (16–28). According to current publications, more than 1/3 of patients with acute COVID-19 develop long COVID-19 symptoms (23, 29–31). The following sections summarize current research on the effects of COVID-19 on physical, cognitive, and mental health, as well as work capacity.

Tuzun et al. (32) concluded that muscle weakness is quite common in hospitalized patients with COVID-19, especially in women with severe disease progression (32). This is supported by results from Paneroni et al. (33), who also found a high prevalence of impairment in muscle strength among hospitalized patients recovering from COVID-19 without previous locomotor disabilities (33). Other studies showed that COVID-19 patients suffered from acute impairments in physical function and capacity (25, 33–35). Aforementioned authors confirmed low (65%), intermediate (13%), and severe (10%) physical function impairments in patients with COVID-19 admitted to a sub-acute care unit (33). Belli et al. (34) demonstrated impairments in physical capacity and activities of daily living in COVID-19 patients in Italy after completion of acute treatment (34). Findings by Martin et al. (25) indicated that hospitalized COVID-19 patients had low functional exercise capacity at discharge and poor recovery after 3 months (25). Curci et al. (35) showed that post-acute COVID-19 patients had low physical capacity. Only 43.6% of the tested post-acute COVID-19 patients were able to walk and only few patients (18.8%) were able to perform the 6-Minute Walk Test (6MWT) with poor results. Furthermore, the authors' findings suggested that post-acute COVID-19 patients suffered from dyspnea and shortness of breath even during minimal activities, resulting in severe disability (35). Initial research results indicated that a COVID-19 infection can also have long-term consequences on physical function, strength, and physical capacity. Results by Bellan et al. (18) showed that 22.3% of tested COVID-19 patients had limited mobility 4 months after discharge (18). Results by Baricich et al. (16) supported this and found impaired physical performance in 32% of the tested COVID-19 patients at 3–6 months after hospital discharge (16). Findings by Blokland et al. (19) revealed that post-COVID-19 patients who were mechanically ventilated suffered from strongly deteriorated cardiorespiratory fitness and decreased peripheral muscle mass (19). Patients who recovered from mild or moderately severe COVID-19 can suffer from muscle weakness at least 12 weeks after diagnosis (28). There are currently few studies investigating the impact of COVID-19 infection on physical activity levels. However, current evidence suggests that physical activity level could decrease after a COVID-19 infection. Delbressine et al. (21) showed that weekly walking time decreased significantly after 3 months compared to pre-COVID-19. Although walking time increased after 6 months compared to 3 months after the onset of symptoms, walking time was still significantly lower compared to pre-COVID-19 (21). Among others, Tanriverdi et al. (28) investigated the physical activity level of recovered COVID-19 patients with mild or moderately severe diseases and showed that, even at least 12 weeks after diagnosis, physical activity level was low in 39.6%, moderate in 33.3%, and high in 27.1% of the participants (28).

Fatigue has evolved as a topic of major interest within the continuous findings of neurocognitive status post-COVID-19 infection. It is already common in other viral infections (e.g., infectious mononucleosis) (36) and may manifest as so-called chronic fatigue syndrome/myalgic encephalomyelitis in patients younger than 60 years with symptoms persisting for at least 6 months (ICD-10-GM-2021-G93.3). Six months after hospitalization, 63% of 1,655 Chinese COVID-19 patients reported fatigue and muscle weakness (23). A meta-analysis of 47,910 COVID-19 patients showed that 80% suffered from long-term symptoms, with 58% specifying fatigue as the first of five most common symptoms (24). In an international cohort study including 56 countries, fatigue ranked first among the most common symptoms reported after 6 months (20). The relevance of fatigue for everyday and workday activities is considered high, not least due to relapses triggered by exercise, physical/mental activity, and stress. Townsend et al. (37) examined predictors for COVID-19 fatigue symptoms and concluded that the occurrence of fatigue is independent from the severity of the initial COVID-19 infection. In addition, clustering of fatigue was noted in women and in cases with a prior diagnosis of depression/anxiety (37). Fatigue in long-COVID cases also appears to manifest together with several other neurological symptoms (20, 38). Common disorders include cognitive functions, such as concentration and memory, headache, muscle pain, continuous sense of smell and taste, or autonomic dysregulation (39). Within 80,000, mostly British, participants studied, infected individuals showed cognitive deficits in working memory, attention, and problem-solving abilities in comparison to persons without a COVID-19 infection (40). These objective deficits are consistent with subjective symptoms, such as reports of brain fog or anomia. Hospitalized, ventilated patients had a mean loss of 7 points on the Great British Intelligence Test, while non-hospitalized, non-treated individuals with confirmed COVID-19 infection also showed poorer cognitive performance compared to a healthy control group. Deficits “could not be explained by differences in age, education or other demographic and socioeconomic variables” [(40), p. 7]. Alemanno et al. (41) reported data from 87 sub-acute patients in an Italian COVID-19 rehabilitation program, 80% of whom had neuropsychological deficits confirmed by low scores on the Montreal Cognitive Assessment (MoCA) and the Mini-Mental-State Examination. They also indicated that cognitive function correlated with age, and that patients partially recovered 1 month after discharge from the hospital (41). Another study examining non-intensively treated COVID-19 patients 5 months after hospital discharge revealed processing speed deficits in 42.1%, delayed verbal recall deficits in 26.3%, and deficits in both processing speed and verbal memory in 21% of the cases (22). Further, there were also correlations found between several pulmonary variables (e.g., arterial oxygen partial pressure) and cognitive abnormalities. The authors concluded that the deficits examined have a major impact on work ability and everyday activities (22).

In addition to neurocognitive symptoms in patients with long-COVID, the results of a meta-analysis found that 45% of hospitalized patients showed symptoms of depression and 47% showed symptoms of anxiety (42). The frequency of depression and anxiety symptoms among COVID-19 patients decreased within several weeks after disease onset but was still present in 9% (26) to 13.6% (17) of patients for anxiety and in ~20% of patients for depression (26). Younger age, working in the healthcare sector, and female gender seem to be risk factors for depression and anxiety (43). Furthermore, 46.9% of ICU patients had symptoms of post-traumatic stress disorder (PTSD), and 23.5% of hospitalized patients had symptoms of PTSD 4–8 weeks after discharge. Obesity was associated with PTSD in the ICU group but not in the (normal) hospitalized group (44). In addition, the following risk factors for PTSD were mentioned: Age, female gender, healthcare professional, and perceived loss of control (27). Healthcare professionals, such as physicians and nurses, are considered particularly vulnerable for developing psychological symptoms after COVID-19 (45–47). Magnavita et al. (48) cited various reasons for the onset of psychological symptoms in COVID-19 infection, e.g., prolonged physiological and social deprivation due to social isolation and quarantine regulations, occupational experience of severe disease progression and mortality (also from colleagues), and persistent risk of infection and/or feelings of guilt from infecting relatives/friends (48).

The literature search resulted in very few studies investigating effects of COVID-19 on work capacity. In a meta-analysis including studies of patients infected with various corona viruses (SARS-CoV, MERS-CoV, SARS-CoV-2), Rogers et al. (27) showed that 446 of 580 patients from a total of 6 studies returned to work after an average of 35.3 months (27). Moreover, the expert group led by Kiekens et al. (49) referred to problems in performing work activities due to COVID-19 and recommended addressing this during follow-up treatment (49). Due to remaining symptoms up to 6 months after acute COVID-19, 45.2% of patients had to decrease working hours while 22.3% were unable to work (20). In the context of resuming work following COVID-19, it should be noted that working in the health care sector during the corona pandemic may have affected the mental health of staff. Lai et al. (50) examined 1,257 healthcare professionals caring for COVID-19 patients in China. These persons showed an increase in symptoms of depression (50.4%), anxiety (44.6%), insomnia (34.0%), and distress (71.5%) (50). Since stress might be a trigger for worsening symptoms, 85% of the patients reported improvements of symptoms with physical and mental training (20).

Taken together, there is a high prevalence of acute and even long-term effects of COVID-19 infection which affects physical, psychological, and cognitive health, as well as work ability. Healthcare professionals showed a particularly high prevalence of COVID-19 infection and are considered a vulnerable group for COVID-19 infection and its acute and long-term consequences. Based on these individual consequences, comprehensive and interdisciplinary rehabilitation of patients is required to recover from COVID-19 exposure and to reduce the long-term effects of COVID-19 on physical and mental health (49, 51). Due to the diversity of the disease progression, e.g., combined with invasive ventilation, complications after ICU treatment, effects on different organ systems, or comorbidities, the condition of post-acute patients at the beginning of rehabilitation varies widely and must be considered when planning and implementing personalized interventions (52). Based on the above-mentioned classification of disease severity, recommendations are made for outpatient (mild, moderate) or inpatient/intensive (severe, critical) care and further rehabilitative care needs (53). Depending on individual symptomatology, pulmonary, neurological, or cardiac inpatient or outpatient rehabilitation should be considered. The case management system of the German Social Accident Insurance Institution for the health and welfare services (“Berufsgenossenschaft für Gesundheitsdienst und Wohlfahrtspflege, BGW”) (54) additionally takes into account the incapacity to work during ongoing COVID-19 treatment. Several German hospitals provide a “Post-COVID-19-Check” (e.g., BG Hospital Berlin), meaning an interdisciplinary diagnostic procedure to examine the full disease symptomatology and derive individual therapy and rehabilitation concepts.

To date, only few studies have investigated the effects of rehabilitation programs after COVID-19 recovery. According to Belli et al. (34), primary rehabilitation measures after hospitalization revealed a significant positive effect on physical resilience and activities of daily living. Despite these effects, 53.3% of patients continued to show low physical resilience in the lowest range after rehabilitation. Therefore, subsequent measures to increase physical resilience are urgently needed (34). The retrospective analysis of data from cardiopulmonary rehabilitation of 28 COVID-19 patients after acute hospitalization also demonstrated improved physical capacity and subjective well-being after completing rehabilitation (55). The results of a randomized controlled trial by Liu et al. (56) with 72 COVID-19 patients confirmed significant improvements of respiratory parameters, 6MWT, health-related quality of life, and anxiety after 6 weeks of respiratory rehabilitation. However, there were no significant improvements of existing depressive symptoms (56). During the examination of inpatient rehabilitation of 100 COVID-19 patients after acute treatment, long-term outcome measures such as the Barthel Index or sit-to-stand-test correlated with the time spent in intensive care (57). The authors emphasized the usefulness of specialized rehabilitation units for severe COVID-19 infection, since greater improvements in some motor outcomes were found in patients who remained in intensive or prolonged acute care. Puchner et al. (58) also concluded that especially severe to critical COVID-19 patients with predominant needs for mechanical ventilation suffered from prolonged physical and mental limitations (e.g., muscle weakness, fatigue, neuropsychological problems) (58). Despite improvements in pulmonary function and physical performance status after an average of 24 days (± 5 days) of rehabilitation immediately after hospital discharge, some patients still showed limited diffusion capacity and neurological symptoms at the end of rehabilitation. This implies continued impairment for everyday activities or at work. Spielmanns et al. (59) compared the rehabilitation of patients with pulmonary disease with that of COVID-19 patients and found more pronounced impairment in COVID-19 patients with severe and critical courses of infection. Despite similar anthropometric values (e.g., mean age, days of pulmonary rehabilitation) and admission values, COVID-19 patients improved significantly more than pulmonary disease patients during rehabilitation in terms of physical performance and actual well-being (59).

In summary, rehabilitation programs can contribute to a reduction in physical, neurocognitive, and psychological restrictions, thereby stabilizing or even increasing patients' physical and mental performance after recovery from COVID-19. This is a prerequisite for maintaining the ability to work on a long-term basis after COVID-19, which is associated with economic costs. Considering the diverse and variable symptomatology, especially the associated physical limitations, social and occupational participation disorders are to be expected. However, there is currently no research focusing on the nature and extent of returning to work after COVID-19. Consequently, COVID-19 is associated with great challenges for the healthcare systems worldwide. In addition to acute patient treatment, long-term care of COVID-19 patients will be necessary after the acute phase due to illness-related impairments. However, based on the lack of longitudinal data and results, the sustainability of rehabilitation effects could not yet be confirmed. Indeed, since patients with severe to critical COVID-19 seem to benefit from rehabilitation soon after the acute phase of infection, rehabilitation programs are recommended for all hospitalized COVID-19 patients to avoid long-term adverse health outcomes (53, 60). However, inpatient rehabilitation programs should not only be offered directly after hospitalization and temporary discharge, but also weeks and months after the acute phase of COVID-19, depending on the existing symptoms of post-COVID (51). Together with the case control management of health services, rehabilitation should be provided to regain functionality in everyday and workday activities.

In this context, this study examines the following main research questions:

The results of the research questions and the documentation of the inpatient rehabilitation, will allow us to determine which special contents and features are feasible for sustainable rehabilitation management and occupational healthcare. Finally, in light of the current literature, we will derive recommendations when implementing inpatient rehabilitation with patients after acute COVID-19 at the BG Hospital for Occupational Diseases Bad Reichenhall.

Using the planned study protocol, long-term effects of OAC on physical capacity, mental health, and work ability will be assessed and described in detail. With a follow-up period of 12 months after the start of inpatient rehabilitation, the project makes an important contribution to the state of knowledge about the long-term effects of the disease. In addition, this study will present associations between physical capacity, mental health, and work ability, as well as the influence of age, gender, comorbidities, and occupation. Moreover, measures, occurring peculiarities, and the feasibility of the inpatient treatment procedure for patients with OAC will be documented. During and at the end of the project period, (interim) results and findings will be summarized in information brochures and presented at information events for the accident insurance institutions. When interpreting the results, it should be considered that the target group is a selective sample of patients with the diagnosis of COVID-19 as a recognized occupational disease or work-related accident. In Germany, the BG Hospital for Occupational Disease Bad Reichenhall is a clinic specialized in the treatment and rehabilitation of occupational diseases and consequently appropriate for the investigation of this sample group. A randomized, controlled study design cannot be applied in the context of the current research project. Since the progression of COVID-19 is acute and associated with direct health-related consequences, it is ethically and medically irresponsible to deny affected insured persons inpatient rehabilitation over a longer period. Nonetheless, the results of the current study will contribute to further developing recommendations for monitoring and coordinating targeted and tailored inpatient rehabilitation in the context of sustainable rehabilitation management for patients after COVID-19. This can help optimize the procedures and processes of post-COVID rehabilitation with the aim to improve the patients‘ physical and mental health, and to support individual occupational ability. Finally, the results will provide a basis for further research, such as the development of disease-specific, health-promoting, and rehabilitative strategies for the sustainable healthcare of patients with COVID-19.

The studies involving human participants were reviewed and approved by the Ethics Committee of the Chemnitz University of Technology (TU Chemnitz), Faculty of Behavioural and Social Sciences (number V-427-17-KM-COVID-19-18022021) and by the Ethics Committee of the Bavarian State Medical Association (number 21092). The patients/participants provided their written informed consent to participate in this study.

KM: conceptualization, methodology, investigation, writing—original draft, visualization, supervision, project administration, and funding acquisition. KaZ: methodology, investigation, writing—original draft, and editing. TA: investigation, writing—original draft, and editing. AT, IB, A-LS, EW, R-CW, FK, AW, and SM: investigation and writing review and editing. MS: conceptualization, investigation, writing review and editing, and supervision. All authors contributed to the article and approved the submitted version.

This research is funded by the German Social Accident Insurance (Deutsche Gesetzliche Unfallversicherung e.V., DGUV) [grant number: FF-FB 0326]. This funding source had no role in the study design, methods, or in writing the report. The publication of this article was funded by Chemnitz University of Technology.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.