A Novel Older-Adult COVID-19 Severity Decoupling Index to Measure the Transition from Pandemic Mortality to Endemic Resilience in Spain

The COVID-19 pandemic produced a severe and heterogeneous burden across age groups, territories and epidemic phases in Spain. Older adults experienced disproportionately high rates of hospitalization, intensive care unit (ICU) admission and death, particularly during the early pandemic waves when vaccination was unavailable. However, the relationship between surveillance-confirmed cases and severe outcomes changed substantially over time, a phenomenon termed epidemiological decoupling. This study developed and applied a novel Older-adult COVID-19 Severity Decoupling Index (O-CSDI) to weekly COVID-19 surveillance data from the 17 Spanish autonomous communities and the two autonomous cities of Ceuta and Melilla. The integrated dataset covered January 2020 to July 2023 and comprised 3496 autonomous community–week observations and 143 variables obtained from four open-access surveillance sources: ISCIII-RENAVE, Datadista/Ministerio de Sanidad, Our World in Data and the MoMo excess mortality system. However, the primary incidence- and ratio-based O-CSDI analyses were restricted to March 2020–March 2022 because open-access individual event counts were no longer structurally comparable after the transition to reduced surveillance. Data from later periods were retained only for contextual vaccination and MoMo mortality indicators. The O-CSDI quantifies the degree to which reported COVID-19 incidence became disconnected from hospitalization, ICU admission and mortality in adults aged 60–69, 70–79 and ≥80 years. The primary version of the index was based on the death-to-case ratio, while complementary versions examined hospitalization-to-case, ICU-to-hospitalization and death-to-hospitalization transitions. Results showed progressive but heterogeneous decoupling across phases, territorial units and age strata, with the strongest decoupling observed during the Omicron BA.1/BA.2 period and the highest residual mortality burden in adults aged ≥80 years. All results were interpreted as ecological, descriptive indicators rather than individual-level risks or causal estimates. These findings provide a reproducible epidemiological tool for retrospective pandemic evaluation and preparedness in ageing populations.