The human immune system is a complex network of cells, organs, and proteins that defends the body against infection and disease. When this defense system is working below its optimal capacity, an individual is described as being immunocompromised or immunodeficient. Understanding the size of this population is a significant public health concern. These individuals are at a higher risk of severe illness and may not respond as effectively to standard public health interventions like vaccines. Establishing a reliable estimate of this population is foundational for optimizing healthcare delivery and influencing public health strategies, from hospital preparedness to community protective measures.
What Being Immunocompromised Means
Being immunocompromised signifies a weakened ability to fight off pathogens like bacteria, viruses, and fungi. This reduced defense can arise from two primary origins: primary or secondary immunodeficiency. Primary immunodeficiencies are relatively rare, inherited conditions caused by genetic defects that affect how the immune system develops or functions. These conditions involve inborn errors of immunity, such as issues with white blood cell production, and include over 550 recognized disorders.
Secondary, or acquired, immunodeficiencies are far more common and develop later in life due to an external factor, such as a disease or medical treatment. Examples include advanced HIV infection, certain types of cancer, or chronic conditions like poorly controlled diabetes or end-stage renal disease. Medical interventions are major drivers, including chemotherapy, high-dose steroids, or immunosuppressive drugs used for organ transplants or autoimmune diseases. Immunocompromised status exists on a continuum, ranging from mild impairment to severe impairment, such as that seen in solid organ transplant recipients.
Difficulties in Measuring Prevalence
Obtaining a single, precise number for the immunocompromised population is a complex challenge rooted in several methodological difficulties. A major hurdle is the lack of mandatory centralized national registries specifically tracking immunosuppression across all causes. Unlike some specific diseases that require reporting, a person’s immunocompromised status is often a secondary clinical factor rather than a primary, reportable diagnosis.
Defining what constitutes “immunocompromised” also introduces variability, as the status is not a simple yes-or-no state. Clinical definitions can vary significantly between different studies, with some focusing only on severe, chronic conditions while others include mild or temporary states. For instance, someone undergoing a short, high-dose course of corticosteroids is temporarily immunosuppressed, but they may not be counted in studies focused on long-term, chronic conditions.
The sheer volume of people taking common medications that cause immunosuppression creates a tracking problem outside of specific disease cohorts. Millions of individuals use biologics or conventional disease-modifying antirheumatic drugs (DMARDs) for autoimmune disorders like rheumatoid arthritis or Crohn’s disease. Unless these individuals are part of a targeted disease registry, their immunosuppressed status due to medication use is often not consistently captured in broad public health data. This reliance on incomplete data contributes to the wide range in published prevalence estimates.
Current Estimates and Major Contributing Factors
Estimates of the immunocompromised population have historically been subject to significant undercounting, often cited as being around 3% of the adult population in the United States. However, more recent and comprehensive analyses suggest this figure is substantially higher. A study using data from the 2021 National Health Interview Survey estimated that approximately 6.6% of U.S. adults, or about one in 15, have current immunosuppression due to health conditions, medications, or medical treatments. This revised figure highlights the growing size and public health relevance of this population.
The largest driver of this prevalence is the increasing use of immunosuppressive medications, particularly for autoimmune conditions. The increase in estimated prevalence between 2013 and 2021 is largely attributable to the greater frequency of prescribing advanced therapies like biologics. These are used for conditions such as inflammatory bowel disease and rheumatoid arthritis, and they intentionally dampen the immune response to control chronic inflammation.
A significant portion of the total estimate is also composed of people with chronic diseases that inherently impair immune function. These conditions include advanced or untreated HIV infection, end-stage renal disease requiring dialysis, and solid organ or stem cell transplant recipients who must take anti-rejection drugs indefinitely. While single conditions like hematological cancer account for small fractions (e.g., 0.1% of U.S. adults), these highly vulnerable groups cumulatively contribute to the overall figure.
Cancer treatment also represents a major, often temporary, factor in the immunocompromised population. Patients undergoing chemotherapy or radiation therapy experience a significant weakening of their immune system as these treatments affect rapidly dividing immune cells. The elderly population, particularly those aged 60 to 69, also show higher rates of immunosuppression, estimated at 9.5% in the U.S. for that age group. This reflects the cumulative effect of age-related immune decline and the increasing prevalence of chronic diseases requiring treatment later in life.