Fig. 5: Estimation of the most likely transition patterns based on the parsimonious patterns of pathogenicity transition of representative A(H1N1)pdm09 strains from 2009 to 2019.

A Estimation of the number and patterns of pathogenicity transitions under the minimum-transition scenario. B Visualization of pathogenicity transitions on the phylogenetic tree, based on the most likely transition patterns. The blue and red lines indicate transitions to be less pathogenic and reversion to be pathogenic, respectively. The phylogenetic tree was constructed based on strains from this study, with the addition of strains from previous studies¹². A black circle in an internal node corresponds to bootstrap values ≥ 70%. The red color denotes strains with high pathogenicity, whereas the black color denotes strains with low pathogenicity. Pathogenicity was determined based on the body weight loss of mice infected with the respective virus, with strains that caused significantly less body weight loss compared with Cal04/2009 being considered to have low pathogenicity.