Survival Advantage of Cohort Participation Attenuates Over Time: Results from Three Long-Standing Community-Based Studies

Abstract Cohort participants usually have lower mortality rates than non-participants, but it is unclear if this survival advantage decreases or increases as cohort studies age. We used a 1975 private census of Washington County, Maryland to compare mortality among cohort participants to non-participants for three cohorts CLUE I, CLUE II, and ARIC, initiated in 1974, 1989, and 1986, respectively. We analyzed mortality risk using time-truncated Cox regression models. Participants had lower mortality risk in the first 10 years of follow-up compared to non-participants [fully adjusted average hazard ratio (95% CI) were 0.72 (0.68, 0.77) in CLUE I, 0.69 (0.65, 0.73) in CLUE II and 0.74 (0.63, 0.86) in ARIC], which persisted over 20 years of follow-up [0.81 (0.78, 0.84) in CLUE I, 0.87 (0.84, 0.91) in CLUE II, and 0.90 (0.83, 0.97) in ARIC]. This lower average hazard for mortality among participants compared to non-participants attenuated with longer follow-up [0.99 (0.96, 1.01) after 30+ years in CLUE I, 1.02 (0.99, 1.05) after 30 years in CLUE II, and 0.95 (0.89, 1.00) after 30+ years in ARIC]. In ARIC, participants who did not attend visits had higher mortality but those who did attend visits had similar mortality to the community. Our results suggest the volunteer selection for mortality in long-standing epidemiologic cohort studies often diminishes as the cohort ages.