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.