• A large number of studies have established a strong association between an inadequate social network and an increased risk of developing a variety of diseases and dying prematurely.
  • One of the major challenges in the fight against infectious diseases such as COVID-19 is therefore to find a balance between the measures necessary to prevent viral transmission while maintaining a sufficient level of social interaction for the mental and physical well-being of the population.

The containment of the population in response to the COVID-19 pandemic has made it possible to substantially reduce the number of people infected with the SARS-CoV-2 coronavirus. According to recent estimates, the measures implemented to contain the epidemic have prevented around 530 million infections worldwide, including 285 million in China and 60 million in the United States. However, these measures mean that less than 4% of the population seems to have been infected with the virus, which means that the fight is far from won and that we must remain vigilant if we want to avoid further waves of infection.

One of the main challenges in the fight against COVID-19 is to find a balance between the measures necessary to prevent viral transmission while maintaining a sufficient level of social interaction for the well-being of the population. Humans are social animals and much has been said, and rightly so, about the deleterious effects of confinement on mental health. This is confirmed by the results of a survey recently published in the Journal of the American Medical Association (JAMA). Using a questionnaire developed to assess the presence of mental disorders (Kessler 6 Psychological Distress Scale), researchers noted that in April 2020, during the COVID-19 epidemic, 14% of American adults exhibited serious symptoms of psychological distress compared to 4% in 2018. These symptoms were particularly common in young adults aged 18 to 29 (24%), as well as among low-income households (less than $35,000 per year).

Physical harm
It should also be remembered that the social environment has a huge influence on physical health in general. It has long been known that certain parameters of our social life, in particular the level of social integration, socioeconomic status and negative experiences at an early age, are among the main predictors of the state of health of individuals and their life expectancy. Disruptions to life in society, such as those caused by a large-scale epidemic, can therefore have negative consequences on the health of the population in the medium and long term.

A large number of studies have established a very clear association between social adversity (negative experiences of life in society) and an increased risk of developing a variety of diseases and dying prematurely (Figure 1). Three main aspects were studied:

Social integration. Studies show that the level of social integration (positive interactions with family, friends and/or colleagues, emotional and physical support from those around them) increases people’s life expectancy by 30 to 80% (Fig. 1B). Conversely, poor social integration (also called social isolation) is associated with an increased risk of several diseases, in particular cardiovascular disease (Fig. 1E), and an increase of about 50% of overall mortality, a risk similar to that associated with well-known risk factors such as obesity, hypertension or sedentary lifestyle (see also our article on this subject). This impact of the level of social integration on health appears to be biologically “programmed”, as similar effects have been observed in a large number of social animals, including primates, rodents, whales and horses. On the scale of the evolution of life on Earth, the link between the degree of social integration and life expectancy has therefore existed for several million years and can consequently be considered as a fundamental characteristic of the life of several species, including ours.

Socioeconomic status. Another consequence of social distancing measures is to disrupt economic activity and, at the same time, cause a drop in or even a loss of income for many people. It has long been known that there is a close correlation between socioeconomic inequalities (generally measured by household income) and the health of the population. For example,  as early as the 1930s, it was observed in the United Kingdom that the risk of death from cardiovascular disease was twice as high among men of lower social class compared to those of the upper classes. Studies since that time have shown that these income differences are associated with an increased prevalence of a large number of diseases (Fig. 1D) and a significant decrease in life expectancy (Figure 1A). In the United States, a comparison of the poorest 1% of the population to the richest 1% of the population indicates that the difference in longevity is of the order of 15 years for men and 10 years for women. This difference may be less pronounced in countries with a better social safety net than Americans (such as Canada), but nevertheless remains significant. In Montreal, for example, the life expectancy of residents of Hochelaga-Maisonneuve was 74.2 years in 2006–2008, compared to 85.0 years for residents of Saint-Laurent, a gap of almost 11 years.

Negative experiences of childhood. The first years of life represent a period of extreme vulnerability to the external environment, both physical and social. One of the dangers associated with periods of prolonged confinement is exposing some children living in precarious conditions to an increased risk of injuries. Unfortunately, this appears to be the case with the COVID-19 epidemic, as U.S. pediatricians recently reported an abnormal rise in children admitted to hospital with severe physical trauma.

This is an extremely worrying situation, as it has been clearly shown that social adversity at an early age is associated with an increased risk of several diseases, including cardiovascular disease, stroke, respiratory disease and cancer (Fig. 1F), as well as a greater susceptibility to viral infections and premature mortality (Fig. 1C). These negative impacts that occur during childhood appear to form a lasting imprint that persists throughout life, even when there is an improvement in living conditions. For example, a study of American doctors reported that subjects who had lived in early childhood in a family with low socioeconomic status had a twice as high risk of premature cardiovascular disease (before age 50), even if they had achieved high socioeconomic status in adulthood.

Figure 1. Association between social adversity and the risk of disease and premature death. (A) Life expectancy at age 40 for American men and women by annual income. (B) Proportion of subjects alive after 9 years of follow-up according to the social network index (quantity and quality of social relations) (n = 6298 people). (C) Average age at death based on the number of adverse childhood experiences (ACEs) (n = 17,337 people). (D) Prevalence of various diseases among American adults as a function of their annual income (n = 242,501 people). (E) Risk of disease by level of social integration among American adults (n = 18,716 people). (F) Risk of disease based on the number of adverse childhood experiences (ACEs) (n = 9,508 people). From Snyder-Mackler et al. (2020).

Role of chronic stress
Several studies indicate that stress plays an important role in the association between social adversity and the increased risk of disease and premature death. All forms of social adversity, whether it is social isolation, insufficient income to meet children’s needs or childhood trauma, are perceived by the body as a form of aggression and therefore cause activation of physiological mechanisms involved in the stress response, such as the secretion of cortisol and adrenaline. For example, exposure to some form of social adversity has been shown to be associated with epigenetic changes (DNA methylation) that alter the expression of certain inflammatory genes involved in the stress response. Studies also show that individuals who are socially isolated tend to adopt behaviours that are more harmful to health (smoking, sedentary lifestyle, excessive drinking, etc.), which obviously contributes to reducing life expectancy.

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