Figure 1.

Pandemic GII.4 variants arise years before pandemic emergence and do not arise from preceding pandemic variants. (A) Temporal MCC tree of GII.4 VP1 sequences from major pandemic and epidemic variants. Variants diverge from all other sampled variants years before their emergence as a pandemic or epidemic (represented by the shaded area). Long branches throughout the tree indicate a high level of unsampled diversity through time. Posterior supports are shown on trunk nodes. (B) Proportion of GII.4 outbreaks in the USA caused by each pandemic GII.4 variant between 2009 and 2018, using outbreak data from CaliciNet (Vega et al. 2011). Sydney 2012 was first detected at low-level in 2011 then rapidly emerged to replace New Orleans 2009 in 2012.