Figure 2

A phylogenetic tree was constructed based on the S1 gene from the 206 isolated field strains and 42 published reference strains using the neighbor-joining method and setting bootstrap 1000 replicates. The vaccine strains that are used in China are marked with ‘’. The Chinese reference strains are marked with ‘
’. The foreign reference strains are marked with ‘
’. Two variant strains that were the result of recombination were isolated and marked with ‘
’. Unmarked strains are field strains isolated in this study.