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 morbid associations on health status, the synergy of their effects, and the mechanisms by which they aggregate. Some morbid associations, due to their significant impact on health, their multiplicative synergistic effects, and shared risk factors, warrant greater attention.

The initial study (1) that led to the formalization of the burden assessment approach described in this article was based on data from two population-based surveys (Health and Social Protection Survey ESPS 2010–14 and Handicap-Household Health Survey HSM 2008) covering more than 60 conditions.

The consequences of these conditions have been studied in terms of activity limitation, perceived health, and mortality—the three main criteria typically used to estimate the burden of disease. However, measuring and characterizing the burden of multimorbidity represented a new approach.

The second step, as presented in our recently published article, was to characterize the epidemiology of multimorbidity in the French adult population and to explore its diversity by gender, age, and several socioeconomic and territorial indicators.

The approach began by identifying conditions that have significant implications in terms of activity limitations, perceived health status, or mortality. Forty-eight chronic conditions independently affecting one of these indicators were selected. We then characterized each group (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 maintain causal relationships with one another or can be explained by common risk factors (obesity, physical inactivity, etc.). Finally, we evaluated the impact as well as the cumulative and synergistic effects of these associations to identify those with the most deleterious effects on health status. These include complicated diseases (such as organ failure), but also conditions affecting the sensory organs and the musculoskeletal system (for activity limitations), as well as mental illness (for perceived health). The synergistic effects of associated conditions range from addition (the effects simply add up between the conditions when they occur in pairs or triads: cardiometabolic disorders, low back pain, osteoporosis, trauma sequelae, depression, and anxiety) to multiplication (the effects are multiplied between the conditions when they occur together: obesity, COPD, migraine, and certain osteoarticular conditions).

The prevalence of multimorbidity in France has been estimated at 30%, based on the co-occurrence of conditions that affect health status (perceived health, activity limitations, or mortality) and using as a criterion the presence of two chronic diseases in the same individual during the year. This prevalence rises to 39% if we use as a criterion the presence of two chronic diseases at any point in a person’s lifetime. However, the prevalence of multimorbidity is higher (+23% to +31%) and occurs earlier (5 to 15 years) in women than in men; it is higher in older individuals, but not negligible in those aged 35 to 44 years (11% and 20% in men and women in this age group, respectively); and it is higher in individuals with the lowest levels of education and income (+84% to +104%), as well as among manual workers (+96%).

A particularly clear inverse dose-response relationship was observed with education level, suggesting a causal relationship. However, the influence of geographic and territorial indicators was very small and negligible once socioeconomic factors were taken into account. Additional analyses identified multimorbid associations particularly marked by social inequalities, which included low back pain, osteoarthritis, COPD, and anxiety.

Santé Publique France has made multimorbidity a priority area in the surveillance and prevention of chronic diseases.

The next step will be to assess evolving trends in multimorbidity in relation to health inequity based on 2022 data from the Drees Autonomy survey and the SNDS survey (National Health Data System), which will allow us to further explore certain morbid associations.

Similarly, the relevance of multimorbidity as a predictor of adverse health outcomes will be evaluated through the risk assessment of COVID-19 and severe COVID-19 in relation to multimorbidity and its components: to this end, a systematic review in collaboration with European partners is currently underway within the agency.

The initial results already allow us to emphasize that monitoring and prevention of multimorbidity should begin as early as midlife and likely even sooner in disadvantaged groups. Particular attention should be paid to the main basic groupings of two, three, or four conditions, which affect 75% of people with multimorbidity in France, especially the hypertension-low back pain combinations (ranked 1st in France) and those involving obesity-hypertension (ranked 2nd), which generally share the same risk factors. The series of associations between common chronic pain conditions (migraine, low back pain, osteoarthritis of peripheral joints) are also widely linked to the same determinants (obesity and mental illness) and carry the same risks regarding the (over)consumption of analgesics. The same is true for the main associations affected by socioeconomic health inequity (involving low back pain, COPD, and anxiety), for which educational determinants appear to be causal.

These results support the prevention and health promotion strategy currently under development at Santé publique France, which recommends taking action before midlife (ages 40–55) to promote health-positive behaviors and environments that reduce the burden of chronic diseases and multimorbidity, thereby supporting healthy aging.