Pneumonia at Marine Corps Recruit Depots: Current Trends in Ambulatory Encounters and Inpatient Discharges

Acute respiratory infections are recognized as a significant source of morbidity for military populations, particularly for recruits. This analysis aims to describe the pneumonia burden at Marine Corps Recruit Depots (MCRD) in Parris Island and San Diego during 2007-2014, as these two depots maintain noteworthy comparisons in vaccine and prophylaxis policies. First, both depots reinstated the adenovirus vaccine in October 2011. Second, San Diego provides the pneumococcal polysaccharide vaccine to all recruits within the first 2 days of arrival, although Parris Island does not routinely vaccinate for Streptococcus pneumoniae. Third, recruits at San Diego routinely receive three doses of penicillin G benzathine for group A Streptococcus bacterium prophylaxis, although those at Parris Island receive one dose year-round and a second dose during the winter months when group A Streptococcus bacterium burden is expected to increase.

Monthly pneumonia rates were estimated using diagnostic codes from ambulatory encounters and inpatient discharge records, and specific causative organisms were assessed using code extenders within the International Classification of Diseases, Ninth Revision. Regression analyses and Spearman's correlation rank tests were used to describe significant trends and the relationship between ambulatory and inpatient rates at each depot.

Although our results indicate the majority of ambulatory encounters and inpatient discharges are attributed to unspecified pneumonia diagnostic codes at both locations, these data still lend noteworthy trends. At both locations, linear trends in ambulatory pneumonia rates significantly declined over the 8-year period, whereas inpatient rates demonstrated less variability and did not significantly decline. Both depots experienced prolonged, heightened pneumonia trends from early 2009-2010, a period which included the global influenza pandemic. Following reimplementation of the adenovirus vaccine during October 2011, the average ambulatory rates at MCRD San Diego (38.02 per 1,000 recruit-months vs. 65.59 per 1,000 recruit-months) and MCRD Parris Island (10.9 per 1,000 recruit-months vs. 22.8 per 1,000 recruit-months) were approximately half the average rate before utilization of the adenovirus vaccine. At MCRD San Diego, a weak correlation between monthly inpatient and ambulatory pneumonia rates suggests that trends for potentially severe pneumonia do not follow those for generalized disease (r^s = 0.43; p < 0.05), whereas correlation results at MCRD Parris Island indicate these monthly trends are positively associated (r^s = 0.71; p < 0.05).

These observations underscore the evidence that pneumonia burden among military recruits is not associated with one single etiology. Recruits are at risk for a range of etiologic agents, and control measures should include a combination of specific medical countermeasures that focus on a single bacterial or viral disease as well as nonmedical public health measures that reduce the overall burden of circulating infectious respiratory agents. The trends described in this report, coupled with the similarities and dissimilarities for public health prevention practices at each depot, may warrant further investigation for a systematic review of social and environmental factors within recruit populations at these two locations.