Data and sample
This study utilized the second wave (Follow-up 1) of the main Canadian Longitudinal Study on Aging (CLSA), and the COVID Baseline (COVID-B) and Exit (COVID-E) survey waves. The CLSA is a national-level population-based longitudinal study with an original baseline sample of 51,338 participants aged between 45 and 85 years when recruited between 2011 and 2015. Two cohorts form the CLSA participants, including the Comprehensive cohort who were randomly selected from the population within 25 km (50 km for lower population density area) of the 11 data collection sites across Canada, and the Tracking cohort randomly selected from the ten provinces of Canada through telephone interview. The Follow-up 1 wave was collected between 2015 and 2018 and included 44,817 participants from Baseline. A total of 28,559 CLSA participants took part in the CLSA COVID-B study (April–May, 2020) after the outbreak of the pandemic, with 24,114 participating in the Exit survey (September–December, 2020). Among participants unable to accept the invitation to take part in the COVID survey, 2500 had died, 3406 withdrew from the CLSA study, 2414 had outdated contact information or did not participate due to other administrative reasons, and 318 required a proxy to participate in the study deeming them ineligible. Data were collected either by email via a web questionnaire (n = 34,498) or by telephone (n = 8202).
In order to measure functional multimorbidity resilience, only participants who have been in the Comprehensive cohort, who were 65 and over at the time of the Follow-up 1 survey, and only participants self-reporting multimorbidity were included, resulting in a final sub-sample of 9211. Multimorbidity was defined as participants diagnosed with two or more of the following 27 chronic conditions collected in the CLSA Follow-up 1 wave (Alzheimer’s disease, back problems, bowel incontinence, cancer, cataracts, diabetes, epilepsy, glaucoma, heart attack, heart disease, high blood pressure, irritable bowel syndrome, kidney disease, Parkinson’s disease, peripheral vascular disease, lung disease, macular degeneration, multiple sclerosis, osteoarthritis, osteoporosis, migraine headaches, rheumatoid arthritis, stroke, thyroid problem, transient ischemic attack, ulcer, and urinary incontinence). Since the full set of chronic conditions were not collected during the COVID-19 surveys, we used Follow-up 1 data to identify the target group.
Detailed information about the CLSA has been published elsewhere [25, 26, 40]. Researchers can access the de-identified data, and information on weighting through the CLSA website (www.clsa-elcv.ca).
Measurement
Outcome variables
There are two outcome variables: 1) self-reported comprehensive impact of the COVID-19 pandemic (CI), and 2) self-reported personal worry about COVID-19 (PW). Based on a similar measure developed by Cao-Lei and colleagues to measure cognitive appraisals of stressful Impact during other disasters, CI uses responses to the question: “Taking everything about COVID-19 into account, how would you describe the consequences of COVID-19 on you and your household?” [41]. The answers to this question include: “1 = very negative”, “2 = negative”, “3 = no effect”, “4 = positive”, and “5 = very positive,” Due to the ordinal nature of the variable and a positive skew in the distribution, we dichotomized the answers into two levels, including “1 = negative impact” (1 and 2) and “0 = positive/no impact” (3 to 5). CI was measured in an identical way at both the COVID-B and COVID-E waves. PW was collected only in the COVID-E wave, based on the question “How worried are you personally about COVID-19 at present” with a 7-point Likert scale, where “1= not at all worried,” and “7= very worried. We dichotomized this variable into “0=less worried” (1 to 4), and “1 = more worried” (5 to 7).
Independent variables
The primary independent variable is the multimorbidity resilience index (MRI). The MRI was developed using CLSA baseline data and has been shown to have good concurrent validity, having statistically significant positive associations with higher levels of perceived health and sleep quality, less perceived pain, and fewer hospital over-night stays and emergency department visits [31]. The MR measure contains three primary resilience sub-indexes), each of which is measured using three variables capturing positive and negative attribution. The MR functional sub-index (MR-FI) consists of the Older Americans Resources and Services Activities of Daily Living (ADL) Scale, Instrumental Activities of Daily Living (IADL) Scale [42] and Summary Performance Score of Functional Ability Scale [43]. The MR psychological sub-index (MR-PI) consists of the Kessler Psychological Distress K10 Scale [44], Center for Epidemiological Studies Depression (CES-D) Scale [45], and the Diener Satisfaction with Life Scale [46]. The MR social sub-index (MR-SI) consists of the total Medical Outcomes Study (MOS) Social Support Survey score [47], a social participation measure including the frequency of participation in activities with family and friends, and a single item measuring perceived loneliness over the past week (from the CES-D 10). In order to standardize the different measurement levels and ranges, a mapping system was applied to convert all the variables into a score between 0 to 10 [31]. The scores of each of the three index composite measures were summed and divided by three to produce a standard set of sub-index scores (range 0–10). A higher score is interpreted as higher levels of multimorbidity resilience. The total MRI score was calculated by adding the three sub-index scores and dividing by three to produce a measure with the same range (0–10). The correlations among the sub-indexes ranged between approximately .20 (functional and social domains) and .47 (psychological and social domains), indicating that they measure distinct resilience domains (See Supplementary Table 1). The MRI was calculated using the CLSA Follow-up 1 wave to measure pre-pandemic levels.
Covariates
Ten demographic and socio-economic social determinants of health were included in the data analysis [21]. Four fixed variables were extracted from the CLSA Baseline wave, including: sex, country of birth, ethnic status and highest educational attainment (which changed very little over time). Sex was categorized as “female” and “male”. Country of birth was categorized into two groups, including “born in Canada” and “out of Canada.” We used visible minority status as a crude indicator of cultural background: “visible minority” and “non-visible minority” (i.e., Caucasian). Education was regrouped from the original seven categories into three (due to small cases in some categories), including: “no post-secondary education,” “some post-secondary education (diploma and certificate),” and “university degrees (Bachelor and above).” Two covariates were extracted from the CLSA Follow-up 1 wave: marital status and income. Marital status was dichotomized into: “not married (single, widowed or separated),” and “married or in common-law relationship.” Personal income was initially measured at five levels, including less than $20,000, $20,000 to $49,999, $50,000 to $99,999, $100,000 to $149,999, and $150,000 and more, and the last two categories were regrouped into one due to small numbers. The remaining four covariates (age, household size, working status, living area) were measured using the CLSA COVID-B wave. Age was measured using single years of age, which we divided into young-old and old-old categories for comparisons: “65 to 74 years old” and “75 years and older.” Household size was measured by three levels, including: “1 person”, “2 persons”, and “3 persons or more”. Work status was measured as “working” or “non-working” (retired and those not in workforce for other reasons). Finally, rural/urban status was dichotomized as: “rural area” and “urban area.”
Analytic procedure
SPSSX Version 26 was used for all analyses. As an initial step, we examined descriptive patterns in the data for all variables among older adults with multimorbidity by sex (female and male) and age groups (65 to 74 years old and 75 years and older). Sex and age have been identified as important correlates of multimorbidity [19]. Additional bivariate analyses were performed for the CI (COVID-B and COVID-E waves) and PW (COVID-E) based on the demographic and social-economic statuses. Subsequently, two sets of multivariable analyses were performed to examine the relationship between pre-pandemic MR index (and the three sub-indexes, MR-functional sub-index, MR-psychological sub-index, MR-social sub-index) on CI and PW between -B and COVID E waves. First, three sets of logistic regression analyses were conducted to examine the association between MRI scores (Total MRI score and three sub-index scores) and the two outcome variables: CI (COVID-B wave and COVID-E wave separately), and PW (COVID- E wave only). Two models were built for each set of logistic regressions. In the first unadjusted model, only the MRI and sub-index scores were included. In the second adjusted model, all ten demographic and socio-economic covariates were incorporated into the model. The odds ratios [EXP(B)], where EXP = exponential, and B = coefficient for each variable were reported. We also report the model fit in each table using two statistical indicators [− 2 Log Likelihood and Cox & Snell R2], where R2 = pseudo variance explained.
Additionally, since CI was measured at two points in the pandemic, we also performed Generalized Linear Mixed Models (GLMM) [48]. GLMM is specifically developed to conduct longitudinal repeated measure analysis, in this case to examine the comprehensive impact and worry, as well as the change from COVID-19 study Baseline to Exit survey. GLMM adjusts models for the random effects of repeated measuring on the same participants and can estimate both within- and between-subject variability. GLMM is suitable for examining dichotomous outcome variables as used in this study. To account for change in CI between COVID-B to COVID-E waves, the survey wave was included in the model as a covariate. Also, interactive terms between sex and survey wave, as well as age group and survey wave, were included in the models to further explore the associations revealed in the descriptive analyses. Similar to the logistic regression, two models were built for each set of GLMM. In the first model, only the survey wave and the MRI score were included. In the second model, the ten demographic and socio-economic covariates were added. The odds ratios [EXP(B)] were reported for studied variables. The model fit is indicated by the Akaike Information Criterion (AIC) [48], and a lower number indicates a better model fit.
Missing cases were examined through the Little’s MCAR test, indicating the missing pattern was random (p > 0.05). List-wise deletion for all variables was applied during data analyses. We conducted supplementary analyses with the missing values imputed according to the levels of measurement, which replicated the results; therefore, we report the findings based on the untreated data.