Information on demographic (such as age, sex, education, roles in work, work experience, location of the hospital work for, body-height, body weight, smoke habit) and clinical characteristic (such as symptoms at admission, ICU admission, comorbidities) of the HCWs was obtained at enrollment.

The HCWs with severe COVID-19 were asked to participate in the first (from July to August 2020), second (from October to November 2020), and third physical examinations (January 2021) in the Physical Examination Center of Union Hospital (Tongji Medical College, Huazhong University of Science and Technology). As of January 2021, 218, 217, and 209 HCWs had completed the first, second, and third physical examination, respectively, and the longest follow-up time can be up to 1 year since discharge from hospital. In each physical examination, functional fitness tests were performed, and blood samples of the HCWs were collected.

Functional fitness test was evaluated using the SFT which included assessment of muscle strength (30-s arm curl test and 30-s chair stand), flexibility (back scratch test, modified trunk rotation and chair sit-and-reach test), and agility/dynamic balance (stance with eyes and functional reach test) [10]. The reasons for choosing the existing functional fitness test are listed below: (1) the SFT test could comprehensively reflect the physical recovery of the participants with respect to muscle strength, flexibility, and agility/dynamic balance; (2) acceptable reliability (ICC > 0.7) was observed for SFT and other functional fitness test including 6-min walking test and SFT could also applied to other population beyond the elderly [11]; and (3) STF needs less time to perform and was found to be convenient for both the participants and the investigators [15]. The functional fitness test was conducted under the professional guidance of doctors in Physical Medicine and Rehabilitation Department of Union Hospital (Tongji Medical College, Huazhong University of Science and Technology). If the HCWs could not or obviously had difficulty in completing the test, the HCWs were considered to have not recovered their functional fitness by the doctors.

The blood samples were used for the determination of antibodies to SARS-CoV-2 and immunological indicators in the Department of Laboratory Medicine of Union Hospital (Tongji Medical College, Huazhong University of Science and Technology). In this study, N protein was coated on the plate for the serum IgM (100 μl, dilution factor was 1:100) and IgG (100 μl, dilution factor was 1:20) enzyme-linked immunosorbent assay. The ELISA kits of Livzon Diagnostics Inc., Zhuhai, China, were used to evaluate the serum immunoglobulin IgM/IgG antibodies against SARS-CoV-2. The methods of detection were described in detail in previous study [16]. If the titres of antibodies in HCWs were greater than 10, HCWs were considered to have positive antibodies against SARS-CoV-2. We assigned the value 5 to 21 and 16 HCWs with seronegativity of antibodies but no exact antibody titre in the first and third physical examination, respectively. In the second physical examination, some HCWs only tested the seropositivity of antibodies and did not perform the titre tests due to the lack of quantitative detection kits. Therefore, only the antibody titres between the first and third physical examinations were compared in this study.

In the present study, in addition to comparing the levels of cytokines and lymphocyte subsets in HCWs during the three physical examinations, 30 HCWs were also enrolled who were admitted to Union Hospital (Tongji Medical College, Huazhong University of Science and Technology) due to SARS-CoV-2 infection (30 and 28 had levels of cytokines and lymphocyte subsets before discharge, respectively). The immunological indicators of these HCWs were measured in the same laboratory (Department of Laboratory Medicine, Union Hospital) using the same method. The BD™ Cytometric Bead Array (CBA) Human Th1/Th2 cytokine kit was used in the measurement of Th1/Th2 inflammatory cytokines. The levels of cytokine profile (IFN-γ, IL-10, IL-2, IL-4, IL-6, and TNF-α) were quantified by BD cytometric bead array analysis. Flow cytometry was performed using a BD FACSCanto™ (BD Biosciences) in lymphocyte subsets (B cells, CD3+ T cells, CD4+ T cells, CD4+/CD8+ cell ratio, CD8+ T cells, and NK cells) detection, and data were analysed with FCAP version 3.0 software.

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