The estimated number of people age 65 and older with Alzheimer’s dementia comes from an updated study using the latest data from the 2020 projections from the U.S. Census Bureau and the Chicago Health and Aging Project (CHAP), a population-based study of chronic health conditions of older people.216 National estimates of the prevalence of all dementias are not available from CHAP, but they are available from other population-based studies including the Aging, Demographics, and Memory Study (ADAMS), a nationally representative sample of older adults.A2,217-218 Based on estimates from ADAMS, 11% of people age 65 and older in the United States have dementia.219 Millions of Americans are living with Alzheimer’s or other dementias. As the size of the U.S. population age 65 and older continues to increase, the number of Americans with Alzheimer’s or other dementias will grow. Both the number and proportion will escalate rapidly in coming years, as the population of Americans age 65 and older is projected to grow from 58 million in 2021 to 88 million by 2050.212-213 The baby-boom generation (Americans born between 1946 and 1964) has already begun to reach age 65 and beyond,214 the age range of greatest risk of Alzheimer’s dementia215; in fact, the oldest members of the baby-boom generation turn age 75 in 2021. This section reports on the number and proportion of people with Alzheimer’s dementia to describe the magnitude of the burden of Alzheimer’s on the community and health care systems. The prevalence of Alzheimer’s dementia refers to the number and proportion of people in a population who have Alzheimer’s dementia at a given point in time. Incidence refers to the number or rate of new cases per year. Estimates from selected studies on the number and proportion of people with Alzheimer’s or other dementias vary depending on how each study was conducted. Data from several studies are used in this section. FIGURE 2 Created from data from Rajan et al.A1,216 *Percentages do not total 100 due to rounding. 65-74 years: 1.72 million (27.6%) 75-84 years: 2.25 million (36.1%) 85+ years: 2.27 million (36.4%) Number and Ages of People 65 or Older with Alzheimer's Dementia, 2021* Total: 6.2 Million Prevalence 20 Alzheimer’s Association. 2021 Alzheimer’s Disease Facts and Figures. Alzheimers Dement 2021;17(3). Prevalence Estimates The 6.2 million prevalence estimate included in this section refers to people age 65 and older who have Alzheimer’s dementia based on symptoms such as memory loss and difficulty thinking. It is based on data from a single longitudinal study in which all individuals who exhibited the clinical symptoms of Alzheimer's were classified as having Alzheimer’s dementia. Future prevalence estimates will be different. First, the number of Americans with Alzheimer’s dementia (the 6.2 million figure reported here) is likely to be lower. This is because future estimates of Alzheimer’s dementia are more likely to be based on whether individuals have the actual brain changes of Alzheimer’s in addition to whether they have clinical changes in memory, thinking and other cognitive skills. Evidence from biomarkerbased studies11,73,220-222 indicates that some individuals counted as having Alzheimer’s dementia based on symptoms do not have the biological brain changes of Alzheimer’s disease; that is, their dementia is caused by something other than Alzheimer’s disease. Both autopsy studies and clinical trials have found that 15% to 30% of individuals who met the criteria for Alzheimer’s dementia based on symptoms did not have Alzheimer’s-related brain changes.11,73,220-222 Thus, these studies indicate that biomarker-based prevalence estimates — rather than symptom-based prevalence estimates — will likely result in up to a 30% decrease in prevalence estimates of Alzheimer’s dementia. Another potential contributor to a decrease in prevalence estimates of Alzheimer’s dementia is the use of multiple symptom-based longitudinal studies when determining prevalence rather than one.A2,219 Second, the estimate of how many people have Alzheimer’s disease (not just dementia due to Alzheimer’s disease) will be higher. For decades it has been recognized that all individuals with dementia pass through a precursor stage frequently referred to as mild cognitive impairment (MCI). More recently, with the advent of biomarkers that detect elevated levels of the betaamyloid and tau proteins that characterize Alzheimer’s, it is now possible to determine which individuals diagnosed with MCI have MCI due to Alzheimer’s disease. As biomarker-based diagnoses become more common, individuals with MCI due to Alzheimer’s disease will be included in prevalence estimates of the number of Americans with Alzheimer’s disease. As reported in this section, the limited data available to date show that about half of individuals age 65 and older with MCI — roughly 5 million Americans — have MCI due to Alzheimer’s disease. Because MCI develops years before dementia, and potentially affects individuals before age 65, there are likely far more than 5 million Americans with MCI due to Alzheimer’s disease. Lastly, the National Institute on Aging – Alzheimer’s Association (NIA-AA) Framework223 hypothesizes that there is an incipient and silent stage of Alzheimer’s disease in which the individual appears not to have the cognitive symptoms of either MCI or dementia. An estimate of the prevalence of “preclinical” Alzheimer’s disease population has been previously published,224 but we clearly need more research to validate preclinical Alzheimer's and determine how to measure it with biomarkers that conclusively represent Alzheimer’s disease.