Temporal trends in the healthy soldier effect in a cohort of Royal Norwegian Navy servicemen followed for 67 years

Objectives To investigate temporal trends in the 'healthy soldier effect' (HSE) among 28 300 Royal Norwegian Navy servicemen who served during 1950-2004. Methods Standardised mortality ratios (SMRs) for all causes, diseases and external causes were calculated from national rates for the entire study period (1951-2017), and for seven successive follow-up periods after the first recorded day of Naval service, for the overall cohort and for two subgroups: land-based personnel and vessel crews. Poisson regression, expressed as rate ratios, was used to compare all-cause mortality between the subgroups. Results In the overall cohort, SMRs for all-cause mortality increased steadily during the first six 10-year follow-up periods, from 0.52 to 0.94, which was still lower than national rates. After 60 years, the lower mortality compared with national rates was no longer statistically significant (SMR=0.93). Low non-neoplastic disease mortality contributed most to the longevity of the HSE. For neoplastic diseases, there was a mortality deficit only for the first and third 10-year follow-up periods. External-cause mortality rose to national rates after 40 years. An HSE was present among vessel crews, but their total mortality rate was 24% higher than that among land-based personnel, who also showed a longer-lasting HSE. Conclusions The HSE eroded gradually over time but was still present at 60 years of follow-up for all-cause mortality. The effect was strongest and most long-lived for non-neoplastic disease, lasted up to 40 years for external causes, and was relatively short for cancers. Land-based personnel showed stronger and longer-lasting HSE than vessel crews.