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Background Studies have demonstrated that optimal protection against childhood influenza requires two “priming” doses of influenza vaccine in the first season of vaccination. Two doses of influenza vaccine are recommended for US children aged 6 months-8 years who received ≤1 dose in prior seasons. We examined risk of influenza among children fully or partially vaccinated during study seasons and vaccine effectiveness (VE) by the number of priming doses. Methods Analyses included children aged 6 months-17 years enrolled during outpatient visits for acute illness for ≤7 days with cough in the US Influenza Vaccine Effectiveness Network during 2014–2015 through 2017–2018. Participants’ respiratory specimens were tested for influenza by rRT-PCR. Vaccination histories back to birth year were obtained from electronic immunization records. VE was calculated by comparing vaccination odds among influenza-positive cases to test-negative controls, as 100 × (1 − odds ratio) adjusted for season, site, age, high-risk status, and calendar time. Results Of 7,583 children, 6,362 (84%) had received ≥1 dose in their lifetime. Among vaccinated children, 90% were primed prior to the enrollment season, and 80% were primed prior to age 2 years. Most (55%) received two priming doses in their first season. Among children recommended to receive two priming doses in the enrollment season, receipt of two doses vs. one was associated with a lower risk of influenza illness (aOR: 0.60; 95% CL: 0.36, 1.00). VE of ≥1 dose in the enrollment season against any influenza among unprimed children was 53% (95% CL: 36, 66). VE of ≥1 dose in the enrollment season was similar among children primed with one dose in their first season (46%; 95% CL: 34, 55) and among those primed with two doses (46%; 95% CL: 35, 55). Overall results were similar when stratified by age and for A/H3N2 viruses, which predominated during study years. Conclusion Among the US children recommended to receive two priming doses of vaccine in the enrollment season, receipt of two doses provided optimal protection. VE in seasons after the priming did not differ by the number of priming doses. Results were driven by predominance of A/H3N2 viruses and may not be similar for A/H1N1pdm09 or B viruses. Current US influenza vaccine recommendations for children are effective and appropriate. Disclosures All Authors: No reported Disclosures.

open forum infectious diseases

900. Effect of Influenza Vaccine Priming on Current Season Vaccine Effectiveness among Children and Adolescents, US Flu VE Network 2014–2015 Through 2017–2018