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Background Given the substantial burden of influenza in the pediatric population, influenza vaccination with live attenuated influenza vaccines (LAIVs) and/or inactivated influenza vaccines (IIVs) is now recommended for children in an increasing number of countries. In recent seasons, the real-world effectiveness of influenza vaccines has varied substantially. In the 2013/14 and 2015/16 influenza seasons, LAIV demonstrated reduced vaccine effectiveness (VE) against A/H1N1 strains. LAIV and IIVs have also demonstrated variable effectiveness against A/H3N2 strains in recent seasons. This study evaluated LAIV and IIV effectiveness in children between the 2016/17 and 2018/19 seasons. Methods Quadrivalent LAIV (LAIV4) and IIV effectiveness studies conducted in the pediatric population from 2016/17 through 2018/19 were identified from published literature, congress presentations, public health websites and personal communication with national investigators. Studies were excluded if they were from countries where Ann Arbor-backbone LAIV was not available for at least one season during the study period, were from randomized, interventional studies, or contained duplicate data from other publications. Results For the three seasons, point estimates of all-strain VE for children ranged from 20% to 74% for LAIV4 and from –20% to 68% for IIV (Fig 1A). During the same period, VE against A/H3N2 for children ranged from –76% to 74% for LAIV4 and from 3% to 56% for IIV (Fig 1B). Point estimates of VE against A/H1N1 for children were 50% and 90% for LAIV4 and ranged from 24% to 87% for IIV (Fig 1C). For influenza B, VE for children ranged from 31% to 80% for LAIV4 and from –12% to 80% for IIV (Fig 1D). Statistical comparison of LAIV4 and IIV VE across each season was not feasible due to the multivariate nature of each study cohort. Figure 1. 2016–2019 Effectiveness of Inactivated and Live Attenuated Influenza Vaccines by Influenza Strain in Children Conclusion During three recent seasons, LAIV4 and IIV showed similar moderate effectiveness against all influenza strains, A/H1N1 strains, and B strains. VE against A/H3N2 for LAIV4 and IIV was good in 2016/17, but decreased in the 2017/18 and 2018/19 seasons. VE estimates for LAIV4 and IIV overlapped for all strains and each subtype, demonstrating the general comparability of LAIV4 and IIV VE in the seasons between 2016 and 2019. Disclosures Allyn Bandell, PharmD, AstraZeneca (Employee, Shareholder) Raburn Mallory, MD, AstraZeneca (Employee, Shareholder) Christopher S. Ambrose, MD, MBA, AstraZeneca (Employee, Shareholder)

1401. Real-World Effectiveness of Inactivated and Live Attenuated Influenza Vaccines in Children During Three Recent Seasons: 2016–2019