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[Preprint]. 2024 May 22:2023.10.02.23296453. Originally published 2023 Oct 3. [Version 2] doi: 10.1101/2023.10.02.23296453

Figure 4. The proportion of influenza positive samples typed as A(H3N2) increases with antigenic drift.

Figure 4.

A-B. Seasonal A(H3N2) subtype dominance increases with H3 and N2 epitope distance. Seasonal epitope distance is the mean epitope distance between viruses circulating in the current season t versus the prior season (t − 1) or two prior seasons ago (t − 2). Distances were scaled to aid in direct comparison of evolutionary indicators. Point color indicates the dominant influenza A virus (IAV) subtype based on CDC influenza season summary reports (red: A(H3N2), blue: A(H1N1), purple: A(H1N1)pdm09, orange: A(H3N2)/A(H1N1)pdm09 co-dominant), and vertical bands are 95% confidence intervals of regional estimates. Seasonal mean A(H3N2) dominance was fit as a function of H3 or N2 epitope distance using Beta GLMs with 1000 bootstrap resamples. C-D. Regional patterns of influenza type and subtype incidence during two seasons when A(H3N2) was nationally dominant. Pie charts represent the proportion of influenza positive samples typed as A(H3N2) (red), A(H1N1) (blue), or B (green) in each HHS region. The sizes of regional pie charts are proportional to the total number of influenza positive samples. Data for Region 10 (purple) are not available for seasons prior to 2009. C. Widespread A(H3N2) dominance during 2003–2004 after the emergence of a novel antigenic cluster, FU02 (A/Fujian/411/2002-like strains). D. Spatial heterogeneity in subtype circulation during 2007–2008, a season with low A(H3N2) antigenic novelty relative to the prior season.