How period life expectancy can distort our interpretation of mortality crises

Mortality crises, such as pandemics, conflicts, and environmental disasters, have profound short- and long-term implications for population health. Although the period life expectancy at birth is commonly used to quantify the magnitude of these events, its reliance on the synthetic cohort assumption can be misleading when interpreted as an impact on lifespan. This study argues that, in addition to period-based estimates, cohort life expectancy estimates should be obtained for adequate and meaningful interpretations of the lifespan impact of mortality crises. Using data from the United Nations' 2024 World Population Prospects, we illustrate this with an application to the most recent and widespread crisis, the COVID-19 pandemic. We analyze changes in period and cohort life expectancy in six countries: New Zealand, Japan, Italy, the United States, Brazil, and Mexico. We analyze two different cohort-based scenarios: the first assumes that the pandemic's impact is fully absorbed by 2024, while the second allows the pandemic to alter future mortality trends. Despite reductions in period life expectancy of up to 70 months in Mexico, cohort life expectancy estimates reveal considerably smaller reductions of up to 3.7 months with an absorbed impact in 2024 and 4 months with a lasting impact. By incorporating the whole mortality experience of cohorts exposed to a certain crisis, cohort life expectancy provides a more accurate interpretation of lifespan effects, offering the wider public, policymakers, and researchers a clearer understanding of demographic consequences and their implications for public health and pension systems.