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

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. [Figure: see text] [Figure: see text] 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|AstraZeneca: Advisor/Consultant|AstraZeneca: Grant/Research Support|GlaxoSmithKline: Grant/Research Support|Meissa Vaccines: Advisor/Consultant|Merck: Grant/Research Support|Moderna: Advisor/Consultant|Moderna: Grant/Research Support|Pfizer: Advisor/Consultant|Pfizer: Grant/Research Support|Sanofi Pasteur: Advisor/Consultant Mary A. Staat, MD, MPH, CDC: Grant/Research Support|Cepheid: Grant/Research Support|Merck: Grant/Research Support|NIH: Grant/Research Support|Pfizer: Grant/Research Support|Up-To-Date: Honoraria Elizabeth P. Schlaudecker, MD, MPH, Pfizer: Grant/Research Support|Sanofi Pasteur: Advisor/Consultant Rangaraj Selvarangan, BVSc, PhD, D(ABMM), FIDSA, FAAM, Abbott: Honoraria|Altona Diagnostics: Grant/Research Support|Baebies Inc: Advisor/Consultant|BioMerieux: Advisor/Consultant|BioMerieux: Grant/Research Support|Bio-Rad: Grant/Research Support|Cepheid: Grant/Research Support|GSK: Advisor/Consultant|Hologic: Grant/Research Support|Lab Simply: Advisor/Consultant|Luminex: Grant/Research Support Geoffrey A. Weinberg, MD, Merck & Co: Honoraria Natasha B. Halasa, MD, MPH, Merck: Grant/Research Support|Quidell: Grant/Research Support|Quidell: donation of kits|Sanofi: Grant/Research Support|Sanofi: vaccine support