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Background Interim estimates of 2017–2018 influenza vaccine effectiveness (VE) against influenza A(H3N2)-related illness in the United States indicated better protection among young children than among older children and adolescents. We examined VE against influenza A(H3N2) illness during five A(H3N2)-predominant seasons from 2010–2011 through 2016–2017 to investigate differences between VE among younger vs. older children. Methods We analyzed data from 11,736 outpatients aged &amp;lt;18 years with medically attended acute respiratory illnesses enrolled at US Flu VE Network study sites during five influenza A(H3N2)-predominant seasons. Respiratory specimens from all enrollees were tested for influenza viruses using reverse transcription PCR. Children with documented receipt of the recommended number of doses of current season inactivated influenza vaccine at least 14 days before illness onset were considered fully vaccinated; partially vaccinated children and those who received live attenuated influenza vaccine were excluded. Vaccine effectiveness was estimated as 100 × (1 – adjusted odds ratio) from multivariable logistic regression adjusting for study site, age, sex, presence of high-risk medical conditions, and days from illness onset to enrollment comparing odds of vaccination among A(H3N2)-positive cases vs. influenza-negative controls. Results A total of 1,854 influenza A(H3N2) cases and 9,882 influenza-negative controls were included; 494 (28%) influenza A(H3N2) cases and 3,637 (41%) controls were fully vaccinated before illness onset. VE ranged from 26% (95% confidence interval [CI], −17% to 53%) to 60% (38%–75%) among children aged 6 months–4 years and from 9% (−16% to 29%) to 66% (37%–82%) among 5–17 year olds (figure). During 2012–2013 and 2014–2015, A(H3N2) VE estimates were significantly higher among younger compared with older children (P &amp;lt; 0.05); in other seasons before 2017–2018, A(H3N2) VE estimates were similar among younger and older children. Conclusion Higher VE against A(H3N2) viruses in younger vs. older children in some seasons suggests immunologic differences in response to vaccine components. Overall, inactivated influenza vaccine provided moderate protection against A(H3N2)-related illness among children. Disclosures M. L. Jackson, sanofi pasteur: Grant Investigator, Research support. L. A. Jackson, Novartis: Grant Investigator, Research support. R. K. Zimmerman, sanofi pasteur: Grant Investigator, Research support. Pfizer: Grant Investigator, Research support. Merck: Grant Investigator, Research support. M. P. Nowalk, Merck: Grant Investigator, Research support. Pfizer: Grant Investigator, Research support. M. R. Griffin, MedImmune: Grant Investigator, Research support. H. K. Talbot, sanofi pasteur: Investigator, Research grant. Gilead: Investigator, Research grant. MedImmune: Investigator, Research grant. Vaxinnate: Safety Board, none. Seqirus: Safety Board, none. J. J. Treanor, Novartis: Board Member and Consultant, Consulting fee.

988. Effectiveness of Seasonal Influenza Vaccines Against Influenza A(H3N2) Illness Among Children Aged &lt;18 Years, US Flu VE Network, 2010–2018

988. Effectiveness of Seasonal Influenza Vaccines Against Influenza A(H3N2) Illness Among Children Aged <18 Years, US Flu VE Network, 2010–2018