IFN-γ response to M.tb PPD at baseline, and at 3 and 12 months following BCG vaccination, among teenage unvaccinated controls and vaccinees. Control subjects n = 62, vaccinated subjects n = 86. Each box shows the median and 25th and 75th centiles. The "whiskers" show the minimum and maximum values other than outliers which are represented as dots above the "whiskers". Grey line represents threshold of positive IFN-γ response (> 62 pg/ml; twice the limit of detection of the assay (31 pg/ml) ).