HALYs are a combined measure of morbidity and mortality. Mortality is measured by the number of years a person died before expected (Years of Life Lost). Morbidity is measured by the number of years a person lives with a specific disability, weighted by its severity (Years of Life Lived with Disability).
Quality adjusted life years (QALYs) and disability adjusted life years (DALYs) are the most common forms of HALYs, with the two most important differences being (i) DALYs express a loss, while QALYs express a gain and (ii) different weightings are applied to different disabilities.
HALYs may be age-weighted (where a year of life lived at the age of 20 is weighted higher than a year of life lived at age one) or discounted (where a year of life lived in five years is weighed less than a year of life lived now).
HALYs are useful for making health prioritization decisions, since they allow a common metric for comparison, but are intended only as a proxy measure of health outcomes. They are, therefore, of limited use in evaluating interventions that might result in higher (non-health-related) quality of life.
Research is underway to extend the HALY concept to encompass a broader measure of wellbeing. A wellbeing-adjusted life year—sometimes referred to as a WELBY (Davison 2015)—would weigh the value of different states based on a proxy of their overall impact on a composite measure of welfare, rather than just health. There is not yet a standard weighting of the welfare-impact of different states, but advances in the research of subjective wellbeing may change this in future.
Bronsteen, John, Christopher Buccafusco & Jonathan Masur. 2013. Well-being analysis vs. cost-benefit analysis. Duke law journal 62: 1603-1689.
World Health Organization. 2016. Metrics: disability-adjusted life year (DALY).
Davison, Ewan. 2015. Exploring a wellbeing adjusted life year (WELBY).