METHODS
Institutional review board approval for the US Hospitalized Adult Influenza Vaccine Effectiveness Network (HAIVEN) study was obtained from the University of Michigan. Cases were identified from 2017-2018 and 2018-2019 enrollees from a single site of the HAIVEN study, encompassing two major hospital systems in southeast Michigan (Michigan Medicine Hospital, Ann Arbor and Henry Ford Hospital, Detroit). Inclusion criteria for the HAIVEN cohort have been described elsewhere15,19. Briefly, adult patients ≥18 years of age were eligible for participation if they presented to the hospital within 10 days of symptom onset or worsening and had a diagnosis or chief complaint broadly consistent with an acute respiratory illness such as influenza or pneumonia. Patients were prospectively recruited and completed a brief interview and research-assistant performed specimen collection to determine laboratory-confirmed influenza illness. Only individuals who tested positive for influenza were included in our analysis.
All participants were interviewed in-person at the time of study enrollment by research personnel. Data obtained from the electronic health record (EHR) via trained research staff chart review or database queries included demographics and comorbid conditions, acute illness characteristics such as symptom duration/type, laboratory and physiologic measures, and outcomes including ICU admission and hospital length of stay15,19.