Study design and participants. The study is designed as a cross-sectional study based on the data from self-reported questionnaires. A total 501 participants representing all counties in Sweden was included in the current study through convenience sampling method. Inclusion criteria were: (i) infected with COVID-19; (ii) age (≥ 18 years); (iii) being a resident of Sweden; (iv) fluent in Swedish and access to the internet in order to complete the online-survey. Data were collected between the 23th of February and the 1th of April 2022.
Procedures. Participants were recruited through convenience sampling via the largest groups on Facebook focusing on individuals infected with COVID-19 and having long-term COVID-symptoms, COVID-19-related organizations, and primary healthcare centers in Stockholm, Sweden, treating post-COVID patients. The online announcement, including information about the study and a link to web-survey, was distributed via Facebook groups, Swedish COVID-organization, and the Karolinska Institute homepage. Participants could also access the web-survey by scanning Quick Response (QR) code provided during their visit at the primary health care centers. Thereafter, participants answered the survey in the online platform, Research Electronic Data Capture (REDCap), hosted locally at Karolinska institute 22,23.
Ethics approval.
The study was approved by the Swedish national ethical board (dnr 2021-06617-01) and informed consent was obtained from all participants. All procedures utilized in collecting data for the current paper follow the ethical standards of the Helsinki Declaration of 1964 and subsequent amendments 24.
Measures. Contextual factors. Contextual factors in the current study consisted of personal factors including age, sex, education level, marital status, work status, and economic status and environmental factors including vaccinated against COVID-19, time of first infection, receiving treatment for post-COVID complaints, and hospitalization for COVID-19. Vaccinated against COVID-19 was measured with single item in which respondents indicated on a 4-point scale if they have received vaccine against COVID-19 (the first dose, the second dose, the third dose, vaccination not received). Time of first infection was measured with single item in which respondents were asked to report date of their infection/infections/positive COVID-19 test/tests (year-month). Receiving treatments for post-COVID complaints was measured using a single item in which respondents stated on a binary scale if they have/had received any treatment for their long-term symptoms after recovery from COVID-19 (yes, no). Hospitalization for COVID-19 was measured using a single item in which respondents stated on a binary scale if they have/had been hospitalized because of COVID-19 (yes, no).
Factors related to body functions and structures. Factors related to body functions and structures included infection with COVID-19, being a high-risk group for COVID-19, and severity of COVID-19 infection in the acute phase. Infection with COVID-19 was measured by a single item in which respondents stated on a 4-point scale if they have/had a confirmed COVID-19 infection supported by positive tests for COVID-19 virus (PCR) and/or positive rapid antigen test on a 4-point scale (I have had it one time, I have had it two times, I have had it more than two times, I believe I've had it, but have not had it confirmed). Being a high-risk group for COVID-19 was measured with a single item in which respondents stated on a binary scale (yes, no) if they have/had been at the high-risk group for COVID-19, such as high blood pressure, angina, stroke, heart disease, diabetes, cancer, smoking, respiratory diseases, and impaired immune system.
Severity of COVID-19 infection in the acute phase was measured with 15 items including fever, fatigue, cough, loss of smell and taste, difficulty breathing or shortness of breath, headache/migraine, aches or pain in body, diarrhoea, rash on skin, runny or blocked nose, nausea/vomiting, arrhythmia/palpitations, sore throat, cognitive difficulties such as memory and attention, and mental health problems such as sleep problems, depression and anxiety 25,26. Here participants rated symptoms that they had at the beginning of the infection or infections and those the following 4 weeks on a 4-point scale (0 = no, 1 = mild, 2 = moderate, 3 = severe). The respondents’ answers to 15 symptoms of COVID-19 items were summed up to calculate the severity of COVID-19 infection in the acute phase (range 0–45, α = .77).
Post-COVID impairments. Post-COVID impairments were used as outcome variables and consisted of 54 items. This questionnaire was developed for the purpose of this study, based on previous questionnaires developed to examine long-term impacts of COVID-19 infection such as the ClinFIT COVID-19 20 and the Functional Compass COVID-19 questionnaire 21 and comprehensive literature review of the long-term effects of COVID-19 2,4,5,7,13,14.
We first listed all common self-reported symptoms of post-COVID-19 condition in previous studies and then asked five experts (two psychologists, one physician, one occupation therapist, and one nurse) to review items and determine whether the items are relevant and also appropriate to be used as self-report measures. Based on the experts’ comments, a list of 54 items was finalized. Then, in the pilot phase, 10 individuals infected with COVID-19 supported by positive tests for COVID-19 virus (PCR) and still experiencing symptoms after the onset of infection were asked to answer the items. Then, cognitive interviews were conducted with participants in the pilot study to check whether the participants had any trouble to understand the items and how participants interpreted the items. For some items, a short description was added in order to provide more clarifications. In the final step, 54 items were adopted to the ICF as post-COVID impairments by two experts including 49 items covering b-categories and 5 items covering d-categories 19. B-categories in ICF consist of impairments in body functions including 8 chapters. The current study sought to cover all eight chapters: 1. "Mental Functions" (14 codes); 2. "Sensory Functions and Pain" (14 codes); 3. "Voice and Speech Functions"(one code); 4. "Cardiovascular, Haematological, Immunological and Respiratory Systems" (5 codes); 5. "Function of the Digestive, Metabolic and Endocrine Systems" (8 codes); 6. "Genitourinary and Reproductive Functions" (one code); 7. "Neuromusculoskeltal and Movement-Related Functions" (3 codes); 8. "Functions of the Skin and Related Structures" (3 codes). Impairments related to Chap. 3 to Chap. 8 were considered as impairments in body system functions in the current study. D-categories in ICF consist of impairments in various actions and life areas. In the current study, 5 items were considered as impairments in activities and participation as results of the COVID-19 infection including difficulty taking care of yourself, impaired control of other diseases and drugs, and keep special diet, difficulties in doing housework, impaired work ability/study ability, and difficulty being to leisure activities. In this study, participants were asked to rate their difficulties/problems for each item on a 4-point scale (0 = no problem, 1 = mild problem, 2 = moderate problem, 3 = severe problem) (for a full description of the instrument see supplemental material).
Cronbach's alphas of the post-COVID impairments in the current study were α = .90 for impairments in mental functions, α = .88 for impairments in sensory functions and pain, α = .90 for impairments in body system functions, and α = .84 for impairments in activities and participation. The respondents’ answers to each sub-category of post-COVID impairments were summed up and divided by the number of items in order to calculate mean.
Statistical Analysis. Statistical analysis was performed using the IBM Statistical Software Package of Social Science (SPSS) version 26.0. Descriptive statistics was used for studying characteristics of the study sample. In addition, descriptive statistics of post-COVID impairments based on ICF categories was presented. For purposes of correlation and regression analyses, categorical variables were dichotomized for obtaining balanced distribution among variable categories; education (dichotomous: low education/high education), marital status (dichotomous: not relationship/in relationship), work status (dichotomous: not working/working), economic status (dichotomous: average and below average/above average), vaccinated against COVID-19 (dichotomous: not vaccinated/vaccinated), infected with COVID-19 (dichotomous: not confirmed COVID-19 infection/confirmed COVID-19 infection), and time of first infection (dichotomous: first and second wave of COVID-19 in Sweden/ during the year 2021 and 2022). A series of bivariate analyzes was performed to examine the associations between the related factors and post-COVID-19 impairments in order to identify primary predictors of the regression models via Pearson’s correlation. Candidate variables significantly associated with post-COVID impairment scores were regarded as potential explanatory variables for regression models. For predicting post-COVID outcomes (impairments in mental functions, impairments in sensory functions and pain, impairments in body system functions, and impairments in activities and participation), a series of hierarchical multiple linear regression analyses were performed. The aim was to examine the unique variance accounted for in the post-COVID impairment scores by the groups of variables and estimate the combined role of contextual factors and factors related to body functions and structures in accounting for variance in post-COVID impairments. Variables that were significantly associated with post-COVID impairment scores in the bivariate analyzes were included as three blocks in hierarchical multiple regression models predicting impairments after COVID-19 infection. Three blocks were entered in the following order: personal factors, environmental factors, and factors related to body functions and structures. We ran separate models for each outcome (impairments in mental functions, impairments in sensory functions and pain, impairments in body system functions, and impairments in activities and participation).