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

INTRODUCTION: 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.

MATERIALS AND METHODS: 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.

RESULTS: 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].

CONCLUSIONS: 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.