Changes in the “healthy soldier effect” by time since start of service in the Royal Norwegian Navy

Abstract

Introduction A healthy soldier effect (HSE) in terms of lowered mortality is often seen in cohorts of deployed and non-deployed soldiers when they are compared to the general populations. The HSE is mainly due to selection on good health, which takes place before and during military service, a demand to stay fit during service, and, in some countries, probably also better access to medical services during and after service. This effect was also present in a peacetime cohort of 28,300 military men who served in the Royal Norwegian Navy after the Second World War. We have extended the follow-up of this cohort with an attempt to investigate changes over time in HSE. Methods The cohort consists of 28,351 military men – commissioned officers, non-commissioned officers and petty officers, and enlisted personnel – who served in the Royal Norwegian Navy at some time between 1950 and 2004. The cohort members were born between 1883 and 1984, median was 1953 [interquartile range (IQR) 26 years]. Information on date and underlying cause of death was retrieved by linkage to the Norwegian Cause of Death Registry. The registry is regarded as complete back to 1951. All cohort members were followed for the incidence of death from first day of recorded service in the Navy but no earlier than 1 January 1951, until emigration, death or end of follow-up, which was 31 December 2015. Median age of the cohort members was 21.2 years (IQR: 2.2) at start of follow-up. We calculated standardized mortality ratios (SMRs) for all-causes combined, for neoplastic and non-neoplastic diseases separately, and for external causes based on the entire follow-up and for successive intervals of each person's follow-up period (0–9, 10–19, 20–29, 30–39, 40–49, and 50+ years) since study entry. Results The cohort members were followed up to 65 years, with an average of 38 years. A total of 5586 deaths and 1.08 mill. person-years were accumulated during follow-up. When considered for the full study period, we observed significant mortality reductions for all causes combined, external causes, and non-neoplastic diseases, while the mortality reduction for neoplasms was only bordering on statistical significance. While all-cause mortality deficit fell with time since study entry, a statistically significant deficit was observed for all successive intervals of follow-up time. SMR rose from 0.52 during the first 10 years of follow-up to 0.93 for the period after 50 years since study entry. Low mortality from non-neoplastic diseases gave the strongest contribution to the overall HSE, as the SMRs for this category gradually increased from one-third to four-fifths of the national rates from the first to the last follow-up interval. For neoplastic diseases, the change over time in relative mortality was less linear. For these diseases, there was a statistically significant mortality deficit only for the first and third follow-up intervals. In subsequent intervals, neoplastic mortality rates were similar to reference rates. External cause mortality rose to reference rates after 50 years. Conclusion The healthy soldier effect eroded gradually with increased follow-up, but was still present throughout the (up to) 65-year long follow-up. The effect was strongest for non-neoplastic diseases, lasted up to 50 years for external causes and was relatively short for cancers.