Research Note: A Novel Sullivan Method Projection Framework With Application to Long COVID.

Originally developed for estimating healthy life expectancy, the traditional Sullivan method continues to be a popular tool for obtaining point-in-time estimates of the population impacts of a wide range of health and social conditions. However, except in rare data-intensive cases, the method is subject to stringent stationarity assumptions, which often do not align with real-world conditions and restrict its resulting estimates and applications. In this research note, we present an expansion of the original method to apply within a population projection framework. The Sullivan method projection framework produces estimates that offer new insights about future trends in population health and social arrangements under various demographic and epidemiologic scenarios, such as the percentage of life years that population members can expect to spend with a condition of interest in a time interval under different assumptions. We demonstrate the utility of this framework using the case of long COVID, illustrating both its operation and potential to reveal insights about emergent population health challenges under various theoretically informed scenarios. The traditional Sullivan method provides a summary measure of the present, while its incorporation into a projection framework enables preparation for a variety of potential futures.