06.02.01. What is the estimated prevalence and incidence of dementia and the source of these estimates? | New Zealand

06.02.01. What is the estimated prevalence and incidence of dementia and the source of these estimates? | New Zealand

17 Sep 2022

Prevalence rates reported by the Deloitte Access Economics report in 2017 (Deloitte Access Economics, 2017) are considered the best estimates for dementia prevalence in NZ. In the absence of NZ dementia prevalence studies, the Deloitte estimates are based on the Alzheimer’s Disease International (ADI) World Alzheimer’s report 2015 estimates for Australasia (Prince et al., 2015).

In turn, the ADI prevalence calculation for the population aged 60+ for Australasia was based on a systematic review and meta-analysis that identified four prevalence studies meeting the inclusion criteria – Gisborne 1980’s, Hobart 1980’s, Canberra 1990’s and a prevalence study on Indigenous Australians in the Kimberley region of Western Australia in 2008. The estimated prevalence for the <60 population was based on a 2003 study in Kensington/Chelsea in England.

These estimates were then used to calculate prevalence rates for the total NZ population as well as the 4 main ethnic groups in NZ – European, Māori, Pacific, and Asian – using age-gender relativities. The Deloitte report does comment “While there is evidence that the incidence of dementia may be higher for Māori and Pacific peoples in New Zealand, due to a higher prevalence of cardiovascular risk factors, no data were available to enable modelling of the extent of this difference.” 

Critique of the prevalence estimates for NZ

  • The population sampled in studies used to estimate the Australasia dementia prevalence is not representative.
  • Study location: Only one of the four studies used to calculate Australasia dementia prevalence was a NZ study and this was carried out in Gisborne in 1983.
  • Ethnicity: Three of the studies did not provide a breakdown by ethnicity and the fourth was in an Indigenous Australian population that found a much higher prevalence in their population compared to the Australian population as a whole.
  • Study year: Other than the indigenous Australian study in 2008, the remaining studies used in the prevalence calculation are 25-35 years old and even if the demographics are representative, other variables such as risk factors may not be representative of the current population.
  • Prevalence calculations do not adjust for the differential risk factors for dementia between ethnicities in NZ.
  • The Lancet commission on Dementia prevention and intervention in 2017 identified nine potentially modifiable risk factors for dementia, with each of the risk factors carrying a relative risk 1.4-1.9 times higher compared to those who did not have the risk factor.
  • All these risk factors differentially affect Māori and PI ethnicities compared to Pakeha, and many of them increase the risk of dementia if they are present in midlife so there is an argument to be made that dementia prevalence is underestimated in these populations.

Deloitte Access Economics. (2017). Dementia Economic Impact Report 2016. Available from: https://www2.deloitte.com/nz/en/pages/economics/articles/dementia-economic-impact-report-2016.html.

Prince, M., Wimo. A., Guerchet, M., Ali, G., Wu, Y., Prina, M. (2015). World Alzheimer’s report 2015: The Global Impact of Dementia: An analysis of prevalence, incidence, cost, and trends. London Alzheimer’s Disease International 2015. Available from: https://www.alzint.org/u/WorldAlzheimerReport2015.pdf.