Influenza activity and influenza-associated regression coefficients in Hong Kong, 2001–2011. (a) Weekly influenza activity for seasonal influenza A(H1N1) (yellow areas), A(H3N2) (dark blue areas), B (green areas), A(H1N1)pdm09 (red areas) and 14 influenza seasons (light blue areas). (b) Weekly regression coefficients (solid line) for influenza A(H1N1) with 95% confidence intervals (dashed lines). Purple area indicates the effect of an increase in laboratory capacity during A(H1N1)pdm09. (c) Weekly regression coefficients (solid line) for influenza A(H3N2) with 95% confidence intervals (dashed lines). (d) Weekly regression coefficients (solid line) for influenza B with 95% confidence intervals (dashed lines) (see Supplementary material for further details about the notation). sH1N1, Seasonal influenza A(H1N1); sH3N2, seasonal influenza A(H3N2); pH1N1, influenza A(H1N1)pdm09.