Comparison of clinical features in a population of basic military trainees versus the general department of defense beneficiary population presenting with influenza

Introduction: Upper respiratory tract infection (URI) is a well-documented cause of morbidity, extra expense, and lost training time among basic military trainees (BMTs). The goal of this study was to characterize the clinical presentation of influenza in the BMT population and to better understand how this presentation differs from that of the general Department of Defense (DoD) beneficiary population (non-BMTs). Materials and Methods: Clinical and demographic data were collected in a prospective study that enrolled DoD beneficiaries presenting to medical treatment facilities in San Antonio, Texas, with URI symptoms between January 2005 and March 2011. Vital signs and symptom duration were collected at the time of enrollment along with basic demographic information. Results: Among 4,448 participants enrolled, 466 (10.5%) tested positive for influenza: 198 of 3,103 BMTs (6.4%) vs. 268 of 1,345 non-BMTs (20%) (p < 0.01); 412 of 466 had complete data for nine symptom-related variables. BMTs were more likely to be Caucasian males and younger than non-BMTs. BMTs had a higher temperature at the time of presentation (101.5°F vs. 100.5°F, p < 0.01). BMTs presented less frequently than non-BMTs with chills (79.7% vs. 94.4%, p < 0.01), malaise (62.1% vs. 87.0%, p < 0.01), nausea (30.2% vs. 43.0%, p < 0.01), and vomiting (12.1% vs. 21.7%, p = 0.01). Multiple logistic regression analysis showed that BMTs were less likely to have the four symptoms compared to non-BMTs even after controlling for gender and age (chills: odds ratio [OR] = 0.3, 95% confidence interval [CI] = 0.1–0.6, p < 0.01; malaise: OR = 0.5, 95% CI = 0.3–0.8, p < 0.01; nausea: OR = 0.5, 95% CI = 0.3–0.8, p < 0.01; vomiting: OR = 0.4, 95% CI = 0.2–0.8, p < 0.01). Although there was no difference in the frequency of subjective fever between the two groups, reported duration of fever was significantly shorter in BMTs than non-BMTs: median of 1 day (range 0–10) vs. 2 days (range 0–8) (p < 0.01). BMTs presented with a composite symptom index mean of 6.2 (standard deviation = 1.4) symptoms, whereas non-BMTs presented with a mean of 6.9 (standard deviation = 1.3) symptoms (p < 0.01). Conclusions: The pretest probability of a BMT presenting with URI symptoms having influenza is significantly lower than that for the general DoD beneficiary population. BMTs with influenza presented sooner, with higher fever, and with fewer overall symptoms than the general DoD beneficiary population. These differences are likely attributable to early reporting and response bias and less likely attributed to age. Military efforts to identify BMTs with suspected influenza infection early and to refer them for treatment promptly are efficacious.