1631. Association Between Type-Specific Influenza Circulation and Incidence of Severe Laboratory-confirmed Cases; Which Subtype Is the Most Virulent?

Abstract Background Excess population mortality during winter is most often associated with influenza A(H3N2), particularly “pneumonia and influenza” deaths. We examined differences between influenza types in their association with severe laboratory-confirmed cases, to determine which is the most virulent. Methods We used nine seasons of comprehensive nationwide surveillance data from Greece (from 2010/11 to 2018/19) to examine the association, separately for influenza A(H1N1)pdm09, A(H3N2) and B, between the number of laboratory-confirmed severe cases (intensive care hospitalizations or deaths) and the overall type-specific circulation during the season (expressed as a cumulated incidence proxy—ILI rate times percent positive, summed over the season), using additive Poisson regression. Results During the study period, and for the same level of circulation during a season, influenza A(H1N1)pdm09 was associated with 3.7 times (95% CI 2.7–5.0) more laboratory-confirmed deaths compared with influenza B, and 2.2 times (95% CI 1.6–3.1) more compared with A(H3N2) (Figure 1). Similar differences were observed for intensive care hospitalizations. Laboratory-confirmed A(H1N1)pdm09 severe cases were more often younger (median age 56 years) compared with influenza B or A(H3N2) (median age 64 and 72 years respectively, both P < 0.001) (Figure 2). Conclusion Influenza A(H1N1)pdm09 is associated with many more severe laboratory-confirmed cases; this is likely due to a more typical clinical presentation and younger patient age, leading to more laboratory testing. In contrast A(H3N2) affects older people and presents more atypically, which is less likely to lead to laboratory testing and confirmation. Focusing on laboratory-confirmed cases, although useful in itself, may provide severely biased estimates of the burden of influenza mortality and morbidity. Disclosures All authors: No reported disclosures.