Fig. 1

One plausible transmission tree. The tree was constructed under the assumption of limited mixing rate between students in two dormitories in relation to within building mixing rates (w = 25 %). In the construction, the other possible sources of seasonal A/H3N2 virus (e.g., the two question marked boxes above building 2 and clinic in Fig. 2 of Liu et al. [12]) have been ignored. The thick arrows represent transmissions from Liu et al. [12] while the thin arrows display one of the most likely transmissions