ArticleJournal of clinical epidemiology2026
Exclusion by design: enrollment criteria limit older adult participation in SARS-CoV-2 vaccine clinical trials.
Article in Journal of clinical epidemiology, 2026. The graph could read no effect estimate from its abstract, so it casts no vote on the map. Not yet cited in PubMed.
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Abstract
BACKGROUND AND
objectivesOlder adults (≥65 years) face a disproportionate burden of COVID-19 mortality yet remain underrepresented in clinical trials despite this group accounting for 76% of COVID-19 deaths. Hundreds of SARS-CoV-2 vaccine clinical trials have been registered on ClinicalTrials.gov; however, the extent of older adult representation remains unclear. Our objective was to evaluate the representation of older adults in SARS-CoV-2 vaccine clinical trials registered on ClinicalTrials.gov and identify enrollment criteria that disproportionately limit older adult participation.
methodsWe conducted a retrospective, cross-sectional analysis of 207 global SARS-CoV-2 vaccine clinical trials registered on ClinicalTrials.gov between November 1, 2019, and September 30, 2025. For each trial, we extracted trial characteristics, participant age data, and enrollment criteria. Age distributions were modeled using reported summary statistics to estimate older adult representation. We compared these estimates to the US population age distribution and COVID-19 mortality. To identify contributing factors to underrepresentation, we systematically evaluated trial eligibility criteria for conditions that disproportionately affect older populations. We compared the age-specific prevalence of these conditions in the US population to their use as exclusion criteria across trials. Additionally, we examined whether trials that excluded common chronic conditions enrolled younger participant populations on average.
resultsWe estimate that 63,223 out of 707,378 (9%) of participants across all trials were aged ≥65, despite older adults accounting for 18% of the US population and 76% of COVID-19-related deaths. Although only 35% of studies had explicit criteria for an upper age limit, we identified common study exclusion criteria, such as hypertension, diabetes, and cancer that are more prevalent among older adults and associated with lower mean participant ages when applied.
conclusionThese findings highlight persistent barriers to equitable inclusion of older adults in vaccine research and underscore the need to reconsider exclusion criteria that disproportionately affect this high-risk population to enhance the generalizability and applicability of clinical trial outcomes.
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