1733. Effectiveness of Influenza Vaccination Against Influenza-Associated Emergency Department (ED) Visits and Hospitalizations Among Children With and Without Underlying Medical Conditions, New Vaccine Surveillance Network (NVSN), 2015-2016 through 2019-2020 Influenza Seasons

Abstract Background Children with underlying medical conditions are at higher risk for developing severe influenza illness. Annual influenza vaccination is recommended for all children ≥ 6 months and can protect against severe influenza. Limited studies have examined if vaccine effectiveness (VE) against influenza illness differs in children with underlying conditions compared to those without. Methods We used a test-negative design to assess VE against laboratory-confirmed influenza-associated ED visits or hospitalizations in children 6 months-17 years of age with and without underlying conditions. Children were enrolled at 7 medical centers within NVSN, a prospective respiratory viral surveillance platform in 7 states, during each influenza season from 2015-2016 through 2019-2020. Influenza vaccination was assessed using documentation from state registries or providers. Underlying conditions were abstracted from medical records. We estimated VE by comparing the odds of vaccination ≥ 14 days prior to symptom onset in case patients with influenza compared to test-negative control patients with non-influenza respiratory illness. VE was adjusted for age, study site, and calendar time. Results A total of 15,739 children were included (2,800 cases and 12,939 controls). Among cases, 48% had an underlying condition, and about half (51%) of those conditions were respiratory (Table). Cases with underlying conditions were more likely to be hospitalized (53%) compared to cases without (26%). Among patients with underlying conditions, 38% of cases and 56% of controls were vaccinated (Figure). Overall (ED or hospitalization) VE was 48% (95% CI: 41%, 54%) among children with underlying conditions and 57% (95% CI: 50%, 62%) among children without. VE against hospitalization was 46% (95% CI: 36%, 54%) among children with underlying medical conditions and 59% (95% CI: 48%, 68%) among children without. Conclusion Vaccination reduced the odds of influenza illness in children with and without underlying conditions. VE was similar in children regardless of presence of conditions. Vaccination was sub-optimal in children with and without underlying conditions and continued efforts are needed to improve vaccination uptake in all children ≥ 6 months. Disclosures Marian G. Michaels, MD, MPH, Merck: Grant/Research Support|Viracor: Grant/Research Support John V. Williams, MD, Merck: Grant/Research Support|Quidel: Board Member Janet A. Englund, MD, Ark Biopharma: Advisor/Consultant|A