Descriptive statistics of the antibody ELISA for samples per month and serotype. Box-and-whisker diagram of the measured optical density percent (ODP) per month and serotype. Showing the smallest observation, lower quartile (Q1), median, upper quartile (Q3), and largest observation. In addition outliers according their interquartile range (IQR) and means are displayed. Each circle represents the measured ODP of a sample. The red line represents the threshold for each serotype, i.e. samples are considered negative if the ODP is for O >= 50, for A >= 45, for Asia 1 >= 35 and for C >= 35.