The effect of competing risks on astronaut and cosmonaut mortality

Astronauts and cosmonauts have been reported to be at substantially lower age-specific risk of death from chronic disease (primarily heart disease and cancers) in comparison to the general populations of the United States and Russia, respectively. Yet, both groups have been at greater age-specific risk of death from external causes, mainly due to plane crashes and spacecraft accidents. In this study we tested the hypothesis that the reported reductions in mortality from natural causes result, to some degree, from survival bias created by early deaths from external causes. Statistical comparisons of baseline characteristics between cause-of-death groups showed no significant differences. Cause-specific survival curves showed no difference in long-term mortality from external causes among either astronauts or cosmonauts compared to Kaplan-Meier curves with censoring for competing causes. Cause-specific survival curves for natural causes suggested a possible upward bias in mortality estimates published thus far for both groups of space explorers. Differences in survival between Kaplan-Meier curves and the cause-specific survival curves were 7% and 5% for astronauts and cosmonauts respectively after 55 years. The data do not support the hypothesis that observed reductions in mortality from natural causes are due in whole or in part to bias created by deaths from external causes at young ages. The data imply that reports of cause-specific mortality for astronauts and cosmonauts may in fact systematically overestimate mortality rates, though these findings should be interpreted with caution as the data are thin at the extremes of follow-up time.