The total number of people with a disease or disorder in a given population at a specific point in time or over a defined period.
Prevalence is a fundamental statistical indicator for understanding the real impact of a disease or disorder in a population. In particular, it makes it possible to estimate the number of people living with a pathology at any given time and is a valuable tool for researchers, doctors and public policy makers.
What is prevalence?
Prevalence refers to the total number of people with a disease or disorder in a given population at a particular time or over a defined period of time. It is usually expressed as a percentage or number of cases per 1,000, 10,000 or 100,000 population. For example, if a disease affects 100 people in a city of 10,000, its prevalence is 1%, or 1 in 100 people. This indicator measures the overall weight of a disease in society. Unlike other health statistics, it does not focus only on newly diagnosed cases, but on all people living with the disease, whether recently diagnosed or for several years.
Prevalence and incidence: not to be confused
Prevalence is often confused with incidence, while these two indicators answer different questions.
Incidence measures the number of new cases that emerge in a given period. It provides information about the risk of developing a disease.
Thus, incidence is relevant only for temporary diseases or to study the kinetics (rate of onset) of an acquired disease.
Prevalence, on the other hand, represents the total number of people affected at any given time. It therefore reflects both the number of new cases and the duration of illness.
Take Parkinson's disease, for example. Every year, new patients are diagnosed: this corresponds to the incidence. But many people live with this disease for many years, even decades. The total number of people affected is then the prevalence.
For example, a rare disease may have a low prevalence despite a relatively high incidence if it is of short duration. Conversely, a chronic disease may have a high prevalence even though few new cases are diagnosed each year.
Different types of prevalence
Researchers generally distinguish between two types of prevalence.
Point prevalence
Point prevalence refers to the number of people with a disease on a specific date.
For example, we can measure how many people are living with multiple sclerosis on January 1 of a given year.
Prevalence of period
Period prevalence takes into account all persons who have reported the disease within a specified time interval, such as an entire year.
This approach provides a broader view of the impact of a pathology in the population.
Why is prevalence important in public health?
Prevalence is a strategic indicator for the organization of health systems.
Knowing the number of people living with a disease allows us to anticipate the need for care, health professionals, medical equipment and medical and social support.
In the field of neurological diseases, this information is particularly valuable. Brain pathologies are often chronic and require long-term management. An increase in their prevalence can have important consequences for patients, their families and the health care system as a whole.
Prevalence also helps identify research priorities. When a disease affects an increasing number of people, scientists may have to step up their efforts to better understand its biological mechanisms, improve its diagnosis or develop new treatments.
What factors influence the prevalence of a disease?
The prevalence of a disease is not fixed. It may evolve as a result of several factors.
One of the key determinants is the development of new treatments. When treatment significantly improves survival or quality of life, more people live longer with the disease. Prevalence then increases mechanically.
Diagnostic advances also play an important role. With better tools, some diseases are detected earlier or more frequently, which can lead to an increase in the number of cases.
The demographic characteristics of the population also influence prevalence. Aging, changing lifestyles or environmental factors (pesticides, nicotine, solvents, etc.) can change the frequency of many diseases.
Epidemics can also have an impact on the prevalence of certain diseases. Mathematically, the prevalence between the beginning of the epidemic and the time of observation may increase if the mortality rate is not high.
Finally, awareness-raising campaigns can help to improve disease identification and prevalence estimates.