Background Longitudinal studies suggest dementia incidence has recently declined in many high-income countries, but there is little evidence on likely dementia trends in the future. We forecast the distribution of future dementia incidence using an approach that allows for sampling uncertainty in measuring historical dementia trends, censoring of dementia cases caused by death, as well as uncertainty in future dementia incidence trends. We use the model to estimate and forecast dementia incidence in England and Wales across age groups 50-100 for men and women.

Methods We used population-based panel data linked to the mortality register across wave 1 (2002-2003) to wave 9 (2018-2019) of the English Longitudinal Study of Aging, representing initially non-institutionalised adults ≥50 years. Standard criteria based on cognitive and functional impairment were used to ascertain incident dementia cases. We estimated time trends in the age- and sex-adjusted dementia incidence rates using a multi-state model. We employed the Lee-Carter model to decompose the incidence rates into period and age effects. In a second step, we model the time series of period effects using a state space model that employs the Kalman filter to discriminate between sampling uncertainty and shocks that drive the underlying incidence process. Thereby, we treat the time-dependent variance output by the multi-state model as a restricted, time-dependent parameter for maximum likelihood estimation of underlying incidence states. We use this step to forecast dementia incidence rates for 2019-2029.

Results Adjusting for age and sex and accounting for missing dementia cases due to death, estimated dementia incidence declined from 2002 to 2008, and increased from 2008 to 2019. Based on the local level model which fits the data best, the dementia incidence trend in England and Wales is forecast to be flat in 2019-2029, but with considerable uncertainty about future incidence.

Conclusion The decreasing dementia incidence trend went into reverse in 2008. Despite a recently observed uptick in dementia incidence trend, the forecasted trend will be flat, but with considerable uncertainty that reflects historical fluctuation in dementia incidence trends. Current estimates of future dementia cases in high-income countries may be unrealistically low.