Study design and participants
We analyzed a unique panel database, the Survey of Health, Ageing and Retirement in Europe (SHARE), which serves as a valuable resource for studying the health and socio-economic implications of the pandemic on individuals aged 50 and older[17]. The survey data across countries was collected with a standardised sampling design and survey questionnaire.
We constructed a new cohort using Wave 8 of the SHARE study (conducted from October 2019 to March 2020) and the SHARE Corona Survey (June to August 2021). This cohort includes participants from the baseline survey of Wave 8, which measured depression, as well as participants aged 50 and older who took part in the SHARE Corona Survey in 2021, totaling 32,003 participants. Among them, 2,579 individuals had experienced COVID-19 infection. Of these, 130 (5.04%) had incomplete primary information. Ultimately, our study includes a final sample of 2,449 participants with complete data., of which 1611 had a positive test for the SARS-CoV-2 virus and 452 experienced symptoms that they attributed to the COVID-19 illness but were not tested. Further details can be found in Supplementary Figure S1.
This report follows the STROBE (Strengthening the Reporting of Observational Studies in Epidemiology) reporting guidelines[18].
Measurement of depression
In this study, participants with depression were defined as having a depression symptom score above established cut-off points, which were assessed with the EURO-D mental health scale. It determines the presence of negative affect and somatic symptoms on a scale from 0 (the lowest level of depression) to 12 (the highest), which covers 12 items (depression, pessimism, wishing death, guilt, sleep, interest, irritability, appetite, fatigue, concentration, enjoyment, and tearfulness).
Clinically significant depression is defined as an EURO-D score of 4 or higher, based on validation work conducted among older adults.
We employed the recommended thresholds(EURO-D scored ≥ 4) to identify participants with depression. The measurement of the EURO-D scores was conducted before the outbreak of the pandemic (October 2019 to March 2020). Although the EURO-D score does not provide a definitive diagnosis of depression such as in the DSM-IV or ICD-10, this cut-off score shows good predictive performance.
Long COVID
Participants were asked whether they had experienced any long-term or lingering effects that they attributed to their COVID-19 illness. They were presented with a list of nine symptoms, including fatigue, cough, congestion, shortness of breath, loss of taste or smell, headache, body aches/joint pain, chest or abdominal pain, diarrhea, nausea, confusion, or any other symptoms.
To quantify the extent of symptoms, a count variable was created ranging from 0 (indicating no symptoms) to 9 (representing the maximum number of symptoms). Participants were considered to have long COVID, if they reported one or more of these symptoms during the 12-month follow-up after COVID-19 infection (from June/August 2020 to June/August 2021).
Covariates
Covariates in this study included sociodemographic indicators (gender, age, level of education, living arrangement, work situation, and economic status), body mass index (BMI, mass in kg divided by square of height in meters), smoking history, seven chronic conditions (history of heart attack, high blood pressure or hypertension, stroke or cerebral vascular disease, diabetes or high blood sugar, chronic lung disease, cancer, and chronic kidney disease), and number of chronic conditions. All covariates were collected during Wave 8 of the SHARE (fielded in October 2019-March 2020) or SHARE Corona Survey 1 (June-August 2020)
Gender was measured as a dichotomous variable indicating male or female, and age was classified into two groups (50–79 years and ≥ 80 years). Education level was categorised into three groups according to the International Standard Classification of Education 1997: low education (ISCED-97 levels 0–2), medium education (ISCED-97 levels 3–4), and high education (ISCED-97 levels 5–6). Living arrangements are measured by whether an individual lives with a partner or spouse. Work situations encompassed two groups before the pandemic (not employed VS employed). Economic status was assessed as a binary variable and included two categories: difficulty making ends meet and easily making ends meet at the outbreak of the pandemic (October 2019 to March 2020). BMI was categorised into two groups (< 30 and ≥ 30). Smoking history refers to participants who had a record of smoking cigarettes, cigars, cigarillos, or a pipe daily for at least one year. Chronic conditions were diagnosed by medical doctors or specialists and reported by self or proxy- participants on follow-up questionnaires.
Statistical analyses
Descriptive data for baseline characteristics of participants are presented as mean (standard deviations, SD) values or frequencies (percentage), as appropriate. Chi-squared test used to compare the baseline characteristics. We compared the frequencies and proportions of 9 long COVID symptoms between participants with pre-existing depression and those without pre-existing depression with Chi-squared test. The significance level was a priori at P < 0.05, with P values from all tests being corrected using the Holm-Bonferroni correction[19].
A multilevel (random intercept) hurdle negative binomial model was employed to assessed the impact of pre-existing depression on the risk of long COVID and on the number of symptoms. Among all participants, the average number of symptoms is 2.25, with a variance of 4.32, which implies that the count data is excessively discrete (variance greater than the mean), subject to the assumption of a negative binomial distribution. Country variance was used as a random effect in the model to capture potential differences.
The model is structured in two parts: the zero part estimates the probability of an individual reporting zero symptoms. In contrast, the count part is the count of occurrences given that the event has occurred [20, 21]. In our study, the event refers to long COVID symptoms.
Further, to explore effect modification[22] by age, gender and number of physical diseases, the effect of pre-existing depression was assessed in subgroups of age (50–69, 70+), gender (man, women) and number of physical diseases (< 2, 2+).
Adjusted rate ratios (aOR) with 95% CIs for the risk of long COVID and adjusted rate ratios (aRR) with 95% CIs for the level of symptoms were calculated in the binary model and the count model, respectively.
A multilevel (random intercept) hurdle negative binomial model was conducted using the “mixed_model” function in the GLMMadaptive package in R (version 4.3.2)[23]. All P values were from 2-sided tests and results were deemed statistically significant at P < 0.05.