No systematic review has appeared so far synthesizing the evidence regarding multimorbidity and loneliness, social isolation, or social frailty. Consequently, our aim was to fill this gap. Three electronic databases (PubMed, PsycINFO, and CINAHL) were searched in our study. Observational studies examining the link between multimorbidity and loneliness, social isolation, and social frailty were included, whereas disease-specific samples were excluded. Data extraction included methods, characteristics of the sample, and the main results. A quality assessment was conducted. Two reviewers performed the study selection, data extraction, and quality assessment. In sum, eight studies were included in the final synthesis. Some cross-sectional and longitudinal studies point to an association between multimorbidity and increased levels of loneliness. However, the associations between multimorbidity and social isolation as well as social frailty remain largely underexplored. The quality of the studies included was rather high. In conclusion, most of the included studies showed a link between multimorbidity and increased loneliness. However, there is a lack of studies examining the association between multimorbidity and social isolation as well as social frailty. Future studies are required to shed light on these important associations. This is particularly important in times of the COVID-19 pandemic.
Keywords: COVID-19; SARS-CoV-2; chronic diseases; loneliness; multimorbidity; multiple chronic conditions; social exclusion; social frailty; social isolation.