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Background The factors that influence influenza viral load are poorly understood, but may have important implications for viral transmission and disease severity. We explored the relationship between patient and virus factors on influenza viral load across 4 consecutive influenza seasons. Methods Adult influenza-positive patients presenting to emergency departments in Baltimore, MD and Taipei, Taiwan between 2014 and 2018 were consented and enrolled. Nasopharyngeal (NP) samples and detailed paired data on symptom duration, demographics, and vaccination were collected. Viral load was inferred using the cycle threshold (Ct) values from quantitative real-time RT–PCR assays for 299 samples and influenza subtype was determined. Bivariate and multivariate analyses were conducted. Results Viral load was impacted by both patient and virus characteristics. Older age and shorter duration of symptoms was associated with a higher viral load (age: Ct difference -0.04, P = 0.022. symptoms: each increasing day, Ct difference +1.02, P < 0.001). Seasonal variability was observed, with the highest viral load associated with the 2014–2015 predominant H3N2 subtype (Ct 21.69, P < 0.001), where there was also a vaccine mismatch. Across seasons, H1N1 was associated with a lower viral load than H3N2 (Ct value + 2.66, P < 0.001). There was not association between gender and immunosuppression on viral load. Conclusion Our study demonstrates that both host and virus factors that can affect Ct values and inferred viral loads. As anticipated, higher viral load, were found to be associated with older age and shorter duration of symptoms; interestingly infection with the H3N2 subtype, traditionally associated with more severe disease was also associated with higher viral loads. Future-focused studies are required to better characterize these relationships, and directly evaluate relevance to both transmission and disease severity. Disclosures All authors: No reported disclosures.

1772. Factors that Influence Viral Load in Patients Infected with Influenza Over Multiple Seasons