The phylogenetic trees for these influenza A(H3N2) HA and NA sequences for the period 1997-2006 were rooted against the reference HA and NA sequences obtained from the influenza vaccine strain A/Syd/5/97(H3N2) downloaded from the Los Alamos National Laboratory (LANL) database (http://www.flu. lanl.gov/vaccine/) [36] . This strain was used as it was representative of the influenza A(H3N2) viruses circulating in 1997, i.e. at the start of this Hong Kong influenza A(H3N2) archive. Other available HA and NA sequences for other vaccine strains (Mos/ 10/99, Fuj/411/02, Cal/7/04) were also downloaded and included in all the phylogenetic trees.