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Background Seasonal influenza causes significant annual morbidity and mortality. The effects of yearly exposures on immunity are not clear and recent observations have demonstrated that long lasting protection against a matched strain may not naturally occur. The 2018–2019 influenza season consisted of an initial peak of H1N1 infections followed by a wave of H3N2 infections. These consecutive waves raise questions about how influenza immunity is affected by sequential exposure to different influenza strains. Challenge studies provide a unique opportunity to study this phenomenon. Here we describe a subset of participants who were sequentially infected in two separate challenge studies with wild-type H1N1 and H3N2 viruses. Methods Healthy volunteers completed two sequential influenza challenge studies at the NIH Clinical Center. Participants were inoculated with reverse genetics, cell-based, GMP wild-type influenza viruses, A(H1N1)pdm09 and A(H3N2) strains. Participants remained isolated in the hospital for a minimum of 9 days and were monitored daily for viral shedding and clinical symptoms. After discharge, participants were followed for 2 months. Results Between 2014 and 2017, 14 healthy volunteers were exposed to Influenza A(H1N1) and Influenza A(H3N2). Time between infections ranged from 2 months to 2 years. Thirteen (93%) participants developed confirmed influenza infection after H1N1 challenge and 9 (64%) after H3N2 challenge. Eight (57%) participants developed confirmed infections after both exposures. Variable degrees of symptoms, shedding, and disease severity were observed. Systemic antibody responses to the HA and NA of both H1N1 and H3N2 varied over time during these sequential infections. Conclusion More than half of all participants who completed 2 sequential H1N1 and H3N2 challenge studies demonstrated confirmed infection to both viruses. These sequential infections had varying effects on the disease experienced and the immunity that developed after infection. These observations are important in understanding the impact of sequential exposures on influenza immunity. Disclosures All authors: No reported disclosures.

2750. Sequential Influenza A H1N1 and Influenza A H3N2 Challenge Infections in Healthy Volunteers