Figure 3

Phylogenetic tree of the NS gene segment of Singaporean specimens isolated from 2004 to 2009. The NS sequences of these specimens were aligned together with the NS gene segments of B/Lee/40, B/Yamagata/16/88, B/Victoria/2/87, the vaccine strains from 2004–2013 (Table 1) and the NA gene segments of Singaporean strains isolated in 1991, 1994 and 1998. Owing to the lack of NS gene sequences of recent strains from SEA, the NS alignment also included strains from the United States, Egypt and New Zealand which were isolated post 2000. The 3 different clusters of NS1 were labelled Clades I, II and III.