A/2009 (H1N1) microarray titers (bars) and the fitted mixture distributions (lines).

The data are aggregated as follows: <20, 20–40, 40–80, 80–160, 160–320, and 320–640. Grey and red bars represent pre- and post-pandemic data, respectively. The solid and dashed line represent the immune and infected component distribution, respectively. The cumulative probabilty density of the mixtures below the detection limit of 20 are marked with black dots.