Epidemiology and population health

Association between body mass index and laboratory-confirmed influenza in middle aged and older adults: a prospective cohort study

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Studies conducted during the 2009 influenza A (H1N1) pandemic found that obesity increases the risk of severe influenza including hospitalization and death. In this study, we examined the relationship of BMI with having laboratory-confirmed seasonal influenza and influenza-related respiratory hospitalization.


We linked a cohort of 246,494 adults aged ≥45 years with data on BMI to subsequent laboratory-confirmed influenza notifications and cause-specific hospitalizations from 2006 to 2015. Cox-proportional hazard models were used to estimate the risk of incident laboratory-confirmed influenza and influenza-related respiratory hospitalizations according to BMI, adjusting for age, sex and other covariates.


After 1,840,408 person-years of follow-up, 1891 participants had laboratory-confirmed influenza notifications (crude rate 10.3/10,000 person-years) of whom 623 were hospitalized for a respiratory illness. Compared to those with healthy BMI (22.5 to <25.0 kg/m2), influenza incidence was respectively 27% (adjusted HR [aHR]: 1.27, 95% CI: 1.10–1.46) and 69% (aHR: 1.69, 1.24–2.29) greater among obese (BMI: 30 to <40 kg/m2) and very obese adults (40 to <50 kg/m2). The equivalent aHRs for hospitalization were 1.57 (95% CI: 1.22–2.01) and 4.81 (95% CI: 3.23–7.17). For every 5-unit BMI increase above 22.5 kg/m2, there was a 15% (aHR: 1.15, 95% CI: 1.09–1.22) increase in risk of having a diagnosis of influenza and 42% increase in hospitalization (aHR: 1.42, 95% CI: 1.30–1.60). These trends did not differ between the pandemic year (2009) and other years.


Our results suggest that obese adults have a similar risk of hospitalization for seasonal influenza as adults with cardiovascular disease and diabetes, and should therefore  be equally prioritized for funded interventions such as targeted immunization programs.

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This research was completed using data collected through the 45 and Up Study ( The 45 and Up Study is managed by the Sax Institute in collaboration with major partner Cancer Council NSW; and partners: the National Heart Foundation of Australia (NSW Division); NSW Ministry of Health; NSW Government Family & Community Services—Ageing, Carers and the Disability Council NSW; and the Australian Red Cross Blood Service. We thank the many thousands of people participating in the 45 and Up Study.


This work was supported by National Health and Medical Research Council (NHMRC) grant (Grant number: 1048180). EB, AN, and BL are supported by NHMRC Fellowships.

Author information


  1. School of Public Health and Community Medicine, University of New South Wales (UNSW), Sydney, NSW, Australia

    • S. Karki
    • , D. J. Muscatello
    • , C. R. MacIntyre
    •  & B. Liu
  2. National Centre for Epidemiology and Population Health, Australian National University, Canberra, ACT, Australia

    • E. Banks
  3. The Sax Institute, Sydney, NSW, Australia

    • E. Banks
  4. College of Health Solutions, Arizona State University, Tempe, AZ, USA

    • C. R. MacIntyre
  5. National Centre for Immunization Research and Surveillance, Kids Research Institute, Children’s Hospital at Westmead, Sydney, NSW, Australia

    • P. McIntyre


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Conflict of interest

PM sits on the Australian Government expert advisory committee for immunization. EB chairs the Australian Government Advisory Committee on the Safety of Vaccines. CRM had received funding and/or laboratory testing support for investigator driven research from CSL Biotherapies, Pfizer and Merck but not related to this research. She has participated in advisory boards for Pfizer, Merck and GSK. All other authors declare no conflict of interest.

Corresponding authors

Correspondence to S. Karki or B. Liu.

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