Infectious Disease · Medical Literature
Assessing the relationship between body mass index and influenza-like illness risk and symptom severity among military health system beneficiaries vaccinated against influenza: a pooled prospective cohort study from a randomized trial.
Abstract
Obesity prevalence is rising globally and associated with increased influenza mortality. The relationship between obesity and influenza-like-illness (ILI) incidence and symptom severity is less clear. US Military Health System (MHS) beneficiaries enrolled in a randomized influenza vaccine trial received one of three influenza vaccines and were followed for ILI via weekly surveys throughout one influenza season. Participants were pooled across vaccine arms into a prospective cohort. We compared ILI risk and ILI symptom severity (Flu-PRO) across body mass index (BMI) categories using multivariable regression models. Among the 8,334 participants with >50% response rates to weekly ILI surveys, 42% had an overweight-range BMI and 24% had an obesity-range BMI. The median age was 36.2, and 86% had no baseline comorbidities. Over one quarter of the participants (27%) reported at least one ILI during follow-up, with a median duration of 11 days. Participants with an overweight and obesity-range BMI were more likely to report ILIs [adjusted incidence rate ratio (aIRR) 1.18 (95% confidence interval (CI): 1.05-1.33) and 1.59 (95% CI: 1.37-1.84), respectively]. Participants with obesity also had higher risks of reporting moderate-to-severe symptoms in four Flu-PRO domains: gastrointestinal, respiratory, systemic, and throat [adjusted risk ratio (aRR) of 1.64 (95% CI: 1.01-2.70), 1.33 (95% CI: 1.05-1.67), 1.32 (95% CI: 1.06-1.63), and 1.20 (95% CI 1.02-1.40) respectively]. Higher BMI was associated with ILI incidence and severity. These findings prompt further research into optimal strategies to reduce the impact of ILI in MHS settings and the general population.
Clinical relevance
This study addresses the underexplored relationship between BMI and influenza-like illness (ILI) incidence and symptom severity in a vaccinated population, moving beyond mortality outcomes previously emphasized in the literature. It affects a large, relatively healthy military beneficiary population (86% without baseline comorbidities), making its findings generalizable to working-age adults. As obesity prevalence continues to rise globally, clinicians need actionable data on ILI risk stratification even among vaccinated patients.
Key findings
1. Among 8,334 vaccinated MHS beneficiaries (median age 36.2 years; 86% without baseline comorbidities), 42% had overweight-range BMI and 24% had obesity-range BMI. 2. Over one quarter of participants (27%) reported at least one ILI during follow-up, with a median ILI duration of 11 days. 3. Overweight-range BMI was independently associated with a higher ILI incidence rate (aIRR 1.18; 95% CI: 1.05–1.33) compared to normal BMI. 4. Obesity-range BMI was associated with a substantially higher ILI incidence rate (aIRR 1.59; 95% CI: 1.37–1.84) compared to normal BMI. 5. Participants with obesity had significantly higher risk of moderate-to-severe symptoms across four Flu-PRO domains: gastrointestinal (aRR 1.64; 95% CI: 1.01–2.70), respiratory (aRR 1.33; 95% CI: 1.05–1.67), systemic (aRR 1.32; 95% CI: 1.06–1.63), and throat (aRR 1.20; 95% CI: 1.02–1.40). 6. The dose-response gradient between BMI category and both ILI incidence and multi-domain symptom severity persisted after multivariable adjustment, suggesting an independent effect of adiposity beyond comorbidity burden.
Practice implications
Clinicians should recognize that standard influenza vaccination does not fully mitigate the elevated ILI risk conferred by overweight and obesity, and patients with obesity-range BMI warrant closer surveillance and earlier intervention at ILI onset given their higher likelihood of moderate-to-severe multi-domain symptoms. In military and occupational health settings, BMI should be incorporated into ILI risk stratification frameworks, and consideration should be given to adjunctive strategies—such as high-dose or adjuvanted influenza vaccines—in individuals with obesity, consistent with evolving immunogenicity data in this population. These findings also support further investigation into whether early antiviral therapy or targeted prophylaxis thresholds should be lowered for vaccinated patients with elevated BMI.