Global dementia study: about 10 new cases a year per 1,000 followed, estimate higher in poorer countries

Counts of how many people live with dementia are easy to find. Estimates of how fast new cases arise, drawn from the same kind of data in rich and poor countries alike, are not. An international team has now pooled individual records from 22 long-running studies in 18 countries and arrived at a single figure: about 10 new cases of dementia a year for every 1,000 people followed.

The study, published on October 12 in JAMA Neurology, also puts the rate higher for women than for men, and higher in low- and middle-income countries than in rich ones. Both gaps come with ranges that overlap, so neither is settled, and 22 studies, however carefully combined, cannot stand in for every country on six continents.

Why a global figure for dementia incidence was missing

Dementia is a decline in memory and thinking severe enough to interfere with daily life, and two different numbers describe how common it is. Prevalence counts everyone living with the condition at one time, while incidence is the rate at which new cases appear. Incidence is the harder of the two to measure, because it takes following the same people for years.

Most of that following has been done in wealthy countries, and the authors of the new paper say global dementia incidence estimates have suffered for it: low- and middle-income countries are underrepresented, and studies analyze their data in different ways. Figures from studies that define and count dementia differently cannot simply be added up.

The gap matters because of where dementia is concentrated. A 2019 analysis estimated that about two thirds of people with dementia live in low-income and middle-income countries. Prevention planning leans on estimates of how much dementia is tied to risk factors that can be changed, yet the same analysis noted that most of the data behind those estimates come from high-income countries only.

Reviews of the field have run into the same wall. A systematic review of trends over time searched the literature to the end of 2017, turned up 10,992 articles and kept 43, and only one of the 43, using data from Nigeria, came from a low and middle-income country. The authors of a 2020 pooled study of European and American cohorts ended their paper by calling for checks of whether their results hold in geographically and ethnically diverse populations.

How the global dementia study pooled its data

The new analysis comes from a consortium called COSMIC, short for Cohort Studies of Memory in an International Consortium. Ashleigh Vella is the first author, followed by a long list of investigators from the member studies. When its organizers described the consortium in an earlier paper, they said they wanted to study dementia risk factors more equitably, in poorer countries as well as rich ones.

The cohorts, groups of people followed over time, began enrolling at different dates, with baseline recruitment between January 1990 and December 2011, and follow-up ran through the end of 2021. A study qualified only if it had diagnosed dementia by clinical assessment, which confined the analysis to regions where data on clinically based dementia diagnoses were available. That left individual records from 22 population-based cohorts that assessed the same people repeatedly, with 38,418 participants spanning 18 countries or territories across 6 continents.

Working from individual records, instead of from each study’s published totals, let the team apply one method to everyone. They counted person-years at risk, so that one person followed for ten years contributes ten person-years, as do ten people followed for one year each. They corrected statistically for participants who dropped out, then calculated rates for each age and sex and combined them.

Because the cohorts differ in their mix of ages, their raw rates would not be comparable. The team therefore standardized the rates to the 2023 global age-sex population structure, recalculating each one as if the cohorts had the same balance of ages and sexes as the world in 2023.

What the study found on dementia incidence

Over 287,632 person-years of follow-up, 3001 participants developed dementia. Once standardized, that came to 9.7 new cases per 1,000 person-years, with a range from 7.89 to 11.97. The range marks where the true rate probably lies, given the uncertainty in the data, and it does not describe the spread between studies or countries. Roughly 10 of every 1,000 participants, then, would be expected to develop dementia in a year of follow-up. That is a rate among people enrolled in studies of aging, with the balance of ages and sexes adjusted to match the world’s, so it does not describe a population of every age, children and young adults included.

Split by sex, the rate was 10.6 per 1,000 person-years in women, with a range from 8.61 to 13.11, and 8.6 in men, with a range from 6.94 to 10.72. Split by national income, it was 13.6 in low- and middle-income countries, with a range from 9.67 to 19.24, and 8.3 in high-income countries, with a range from 6.57 to 10.54.

In both comparisons the ranges overlap. The women’s and the men’s share the stretch from 8.61 to 10.72, and the two income groups share a narrower one, from 9.67 to 10.54. The paper reports each rate as higher, but overlap of that kind means the true gap could be smaller than the headline numbers suggest, and the range for poorer countries is especially wide.

These averages also hide the largest influence of all, which is age. In the 2020 study of seven cohorts in Europe and the United States, incidence climbed from about 4 per 1,000 person-years at ages 65 to 69 to 65 per 1,000 person-years at ages 85 to 89. A single global figure of about 10 sits between those extremes because it blends the ages the cohorts covered.

Dementia incidence in low- and middle-income countries

A higher rate in poorer countries runs against much of the older literature. A 2012 study in the Lancet, a British medical journal, noted that most earlier studies from countries with low or middle incomes had reported rates substantially lower than those from countries with high incomes. Its authors, a network called the 10/66 Dementia Research Group, suspected the low rates were partly an artifact of how dementia was defined.

The group followed people aged 65 and older in Cuba, the Dominican Republic, Venezuela, Peru, Mexico and China. Of 12,887 interviewed at the start, 8,137 who were free of dementia were seen again three to five years later, and how much new dementia turned up depended heavily on the definition. Under criteria the group had designed to work across cultures, incidence ran from 18.2 to 30.4 per 1,000 person-years across the sites. Under a stricter standard set of criteria it ran from 9.9 to 15.7, so the first set of rates was 1.4 to 2.7 times the second.

The 10/66 authors also gave a reason to expect more dementia in poorer settings, through an idea called cognitive reserve: education and mentally demanding work build up a capacity that lets the brain cope with damage for longer. Where schooling has been scarce, they reasoned, a higher incidence of dementia might be expected than in high-income countries.

The 2019 analysis of risk factors pointed the same way. It estimated the share of dementia tied to nine risk factors that can be changed, such as less childhood education, high blood pressure, hearing loss and smoking. Worldwide that share was put at 35%, but it came to 39.5% in China, 41.2% in India and 55.8% in a Latin American sample. The authors concluded that the prevention potential in those countries is large, and greater than in high-income countries. So the new estimate of 13.6 against 8.3 fits a line of reasoning that predates it, though it does not prove that reasoning, because differences in how each cohort identified dementia could also contribute.

Dementia incidence in women and men

More women than men live with dementia. A global forecast published in The Lancet Public Health in 2022 estimated that women with dementia outnumbered men with dementia globally in 2019, by a ratio of about 1.7 to 1. Much of that is survival: women live longer, and dementia is a disease of old age.

Whether women develop it at a higher rate at the same age is a separate question, and earlier pooled studies mostly said no. The European and American analysis found rates at each age notably similar between men and women. A 2023 review that assembled 205 studies with close to a million participants in 43 countries reached the same answer, finding no sex differences in age-specific dementia incidence worldwide except at age 90 and over, where women’s rates were higher. That review’s authors raised a caution that applies here as well. Because women live longer than men, they wrote, more dementia may be observed among women, and a difference in survival can pass for a difference in risk.

The new paper’s figures, 10.6 for women and 8.6 for men, are standardized by age. They lean toward a higher rate in women, with overlapping ranges, and are best read as one more data point in a question that remains open.

What the 2050 dementia projection assumes

The team carried its rates forward. Applied to the world population expected in 2050, they gave 24.7 million new cases in that year, a projection made assuming constant incidence over time.

That figure counts new cases in a single year. The 2022 forecast measured something else, the number of people with dementia, which it expected to rise from 57.4 million in 2019 to 152.8 million in 2050 as populations grow and age. What drives that rise differs by region, and in sub-Saharan Africa, population growth contributed most to the increases.

Holding rates constant is a real limit, because rates have been moving. Across the seven European and American cohorts, dementia incidence fell by 13% per calendar decade between 1988 and 2015. Whether anything similar is happening elsewhere is unknown, for lack of data. If rates fall, 24.7 million will prove too high, and if the risk factors that are more common in poorer countries spread, it could prove too low.

What the global dementia estimates cannot show

Twenty-two studies for the world. The cohorts span 18 countries or territories. Most of the world’s countries are not among them, and a cohort drawn from one city or region may not represent its whole country.

Overlapping ranges. The gaps between women and men and between income groups are estimates with wide margins. The low- and middle-income figure, from 9.67 to 19.24, is the least certain.

Diagnosis across settings. Every study used a clinical diagnosis, but clinical practice and assessment tools differ between sites. The consortium itself says it is still developing methods to reliably assess dementia where data are limited, in under-resourced regions.

Older cohorts. Recruitment ran from 1990 to 2011. People who entered a study three decades ago grew up with different schooling, diets and medical care from those reaching old age now.

Who joins and who stays. People who volunteer for long studies, and who remain in them, tend to be healthier than those who do not. The analysis corrected for drop-out statistically, which reduces that problem without removing it.

A projection, not a forecast. The 2050 number holds today’s rates fixed and changes only the population.

All-cause dementia. The figures combine every type of dementia. They say nothing about Alzheimer’s disease or any other cause on its own.

The authors offer their harmonized estimates as a comparable benchmark for global dementia incidence, meant to inform planning for prevention, diagnosis and memory-care services.

Whether memory or thinking changes in a particular person point to dementia is assessed by their doctor.

The figure of about 10 per 1,000 is a starting line drawn from the places that have data, and most of the world’s countries are not yet among them.