Assessing the burden of multimorbidity must go far beyond simply counting chronic diseases, as is most often done in the epidemiological literature. It must take into account the varying impacts of disease combinations on health status, the synergy of their effects, and the mechanisms underlying their aggregation. Certain disease combinations, due to their significant impact on health, their multiplicative synergistic effect, and shared risk factors, deserve greater attention.

The initial studies (1) that led to the formalization of the burden assessment approach described in this article were conducted using data from two population surveys (the Health and Social Protection Survey [ESPS] 2010–14 and the Household Disability and Health Survey [HSM] 2008) covering more than 60 conditions. Their consequences were studied in terms of activity limitation, perceived health, and mortality—the three criteria typically considered in estimating the burden of disease. This represented a new approach to measuring and characterizing the burden of multimorbidity.

The second step presented in the recently published article was to characterize the epidemiology of multimorbidity in the French adult population and to explore its diversity according to sex, age, and several socioeconomic and regional indicators.

The approach first involved identifying conditions that have a significant impact on activity limitations, perceived health status, or mortality. Forty-eight chronic conditions independently affecting one of these indicators were selected. We then characterized the clusters (dyads, triads, etc.) of associated conditions in terms of frequency and the mechanisms explaining their association (shared risk factors, causal relationships, etc.). The most common multimorbid associations include cardiometabolic, musculoskeletal, and mental health conditions, which have causal relationships with one another or can be explained by common risk factors (obesity, insufficient physical activity, etc.). Finally, we assessed the impact and the cumulative and synergistic effects of these associations and then identified those with the most deleterious effects on health status. These include complex diseases (such as organ failure), as well as conditions affecting the sensory organs and the musculoskeletal system (for activity limitations), and mental illnesses (for perceived health). The synergistic effects of associated conditions range from addition (effects simply add up between conditions when they are paired in dyads or triads: cardiometabolic conditions, low back pain, osteoporosis, trauma sequelae, depression, and anxiety) to multiplication (effects multiply between conditions when they are associated: obesity, COPD, migraine, certain osteoarticular conditions).

By identifying combinations of conditions that affect health status (perceived health, activity limitations, or mortality) and using the criterion of an individual having two chronic diseases within a year, the prevalence of multimorbidity in France has been estimated at 30%. This prevalence rises to 39% when the criterion is the presence of two chronic diseases at any point in a person’s lifetime. However, the prevalence of multimorbidity is higher—ranging from 23% to 31%—and begins earlier—by 5 to 15 years—in women than in men; it is higher among older individuals, but not negligible among those aged 35–44 (11% and 20% respectively among men and women in this age group); and it is higher (84% to 104%) among individuals with the lowest levels of education and income and (96%) among manual workers.

A particularly clear inverse dose-response relationship was observed with educational level, suggesting a causal relationship. In contrast, the influence of geographic and territorial indicators was very weak and negligible once socioeconomic factors were taken into account. Additional analyses identified multimorbid conditions particularly marked by social inequalities, including low back pain, osteoarthritis, COPD, and anxiety.

Santé publique France has made multimorbidity a priority area for the surveillance and prevention of chronic diseases.
The next step will be to assess trends in multimorbidity, particularly in relation to health inequalities, using 2022 data from the Drees Autonomy Survey and the SNDS (National Health Data System), which allows for the exploration of certain multimorbid associations.

Similarly, the relevance of multimorbidity as a predictive indicator of adverse health outcomes will be tested by assessing the risk of COVID-19 and severe COVID-19 associated with multimorbidity and its components: a systematic review in collaboration with European partners is currently underway within the agency.

Already, initial results highlight that the monitoring and prevention of multimorbidity should begin as early as midlife and likely even earlier in disadvantaged groups. The main basic clusters of two, three, or four conditions, which affect 75% of multimorbid individuals in France, should be given special attention, particularly the combinations of hypertension and low back pain (the most common in France) and obesity and hypertension (the second most common), which largely share the same determinants, as well as the range of common painful conditions (migraine, low back pain, osteoarthritis of peripheral joints), which share the same determinants (obesity, mental health conditions) and the same risks associated with the (over)use of analgesics. The same applies to the main conditions affected by socioeconomic health inequalities (involving low back pain, COPD, and anxiety) and for which educational determinants appear to be causal.

These findings support the prevention and health promotion strategy currently being developed at Santé publique France, which proposes taking action starting in midlife (ages 40–55) to promote health-promoting behaviors and environments in order to reduce the burden of chronic diseases and multimorbidity, and thereby foster healthy aging.