FIGURE 4.

Phylogentic analysis of NA gene of influenza B-Yamagata lineage virus from severe acute respiratory infection and influenza-like illness cases. The phylogenetic trees were constructed using BEAST program and employing the general-time reversal HKY substitution model/strict clock model with a gamma-distributed rate parameter. The viruses isolated from severe acute respiratory infection cases were shown in red, and the vaccine strains were shown in blue. HA-Y3/NA-V4 reassortants were marked in yellow.