Scientists called epidemiologists study patterns in the way diseases affect groups of people. For instance, epidemiologists are often called in to find out why an outbreak of a viral infection happens in a particular location or why one group of people may be more likely to get a certain disease than another group.
Epidemiologists also estimate how common a disease is. They determine the incidence of a disease, or how many new people are diagnosed with the disease each year, and the prevalence of a disease, or how many people in a population have the disease at any given time.
RA and other types of arthritis
Arthritis is a common health problem in the US population, affecting more than 46 million people and resulting in disability for 19 million people.1 In fact, among chronic diseases in the US, arthritis causes more disability than any other condition, including heart disease, diabetes, and back or spine problems.2 RA is the third most common type of arthritis behind osteoarthritis (prevalence 26.9 million) and gout (prevalence 6.1 million). RA affects approximately 1.3 million in the US.1
How common is RA among autoimmune diseases?
RA is one of the more common autoimmune diseases, with rates higher than a number of other conditions, including psoriasis, Crohn’s disease, Type I (insulin-dependent) diabetes, lupus, and multiple sclerosis.3-5
The prevalence of RA globally and in the US
RA is a common chronic disease that affects about 1% of the world population.6 The prevalence of RA in the US, based on rates of RA from a 1995 Minnesota study and 2005 Census data, is currently estimated at approximately 1.29 million people or 0.6% of the population. This is down from an earlier estimate of 2.1 million people. The prevalence and incidence (new cases per year) of RA appears to have declined since the early 1960s.7 One study conducted in Rochester, Minnesota, found that the incidence of RA had declined for both men and women over a 40-year period.8
Even with these declines, RA occurs at twice the rate in women compared with men, with a prevalence of 1.06% in women (as a percentage of the total population) compared with 0.61% in men.6
Rates of RA in different age groups
The incidence (new cases per year) of RA increases with increasing age in most populations until about the eighth decade of life, when it declines.9 Results from the same Rochester, Minnesota study mentioned above found that the average yearly incidence among different age groups increased until about ages 74-84 and decreased thereafter. The incidence peaked earlier for women than men at about ages 55 to 64 years for women, compared with 75 to 84 years for men. At no point was the yearly incidence of RA higher for men than women.8
Prevalence in different populations
The prevalence of RA varies widely from population to population, with the lowest rates in Asian countries and higher rates among certain Native American populations. This suggests that there is indeed some genetic component underlying susceptibility to the disease.10