RSV in Ontario hospitalized adults who would have been excluded from vaccine trials

This study aimed to describe the proportion of patients hospitalized for respiratory syncytial virus (RSV) infection who would have been excluded from RSV vaccine phase 3 trials, and to compare length-of-stay and in-hospital mortality between patients who would have been included versus excluded from these trials. We performed a multi-center retrospective cohort study across 31 acute care hospitals in Ontario, Canada from 2015 to 2023, before approval of RSV vaccines. Adults admitted to hospital with a diagnosis of RSV infection were categorized as either trial-eligible or ineligible based on phase 3 vaccine trial criteria. Trial-ineligible patients were younger than 60 years or had ≥1 of the following: dementia, organ or hematopoietic stem cell transplant, kidney failure, autoimmune disease, HIV infection, or malignancy. A multivariable competing risk model was used to compare length-of-stay and in-hospital mortality. Of 2496 patients hospitalized with RSV infection, 883 (35.4%) patients would not be eligible for the RSV vaccine trials. The median (interquartile range) hospital length-of-stay was 6.0 (3.1–10.9) and 5.7 (3.4–10.2) days in the trial-ineligible and eligible groups, respectively. In hospital, 86/883 (9.7%) and 90/1613 (5.6%) patients died in the trial-ineligible and eligible groups, respectively. In a competing risk model, the trial-ineligible group had an adjusted sub-distribution hazard of 0.86 (95% CI 0.79–0.95) for discharge alive and 1.76 (95% CI 1.25–2.48) for dying in hospital. A third of hospitalized RSV patients would have been excluded from phase 3 vaccine trials. These patients had longer lengths-of-stay and higher mortality risk.