Table. Proportion of circulating influenza viruses with HA amino acid sequences closely matched with vaccine strains, Hong Kong, China, 1996–2012*.

Year, season	Influenza A(H3N2) vaccine strains, %†			Influenza B vaccine strains, %†
	Northern Hemisphere	Southern Hemisphere		Northern Hemisphere	Southern Hemisphere
1996
Winter	None‡	None‡		All§	All§
Summer	None‡	None‡		77.8	77.8
1997
Winter	All§	None‡		14.7	None‡
Summer	94.4	94.4		None‡	None‡
1998
Winter	None‡	None‡		None‡	None‡
Summer	None‡	None‡		None‡	None‡
1999
Winter	18.8	None‡		6.3	6.3
Summer	None‡	None‡		None‡	None‡
2000
Winter	None‡	None‡		None‡	None‡
Summer	None‡	None‡		None‡	None‡
2001
Winter	All§¶	None‡		None‡	None‡
Summer	84.6¶	84.6¶		None‡	25.0
2002
Winter	41.2¶	41.2¶		None‡	None‡
Summer	6.7	6.7		12.5	12.5
2003
Winter	None‡	None‡		73.3	6.7
Summer	None‡	None‡		None‡	None‡
2004
Winter	None‡	None‡		None‡	None‡
Summer	None‡	None‡		None‡	None‡
2005
Winter	31.3#	None‡		29.4	None‡
Summer	None‡	None‡		12.5	12.5
2006
Winter	None‡	None‡		None‡	None‡
Summer	None‡	None‡		None‡	90.0
2007
Winter	52.4	None‡		9.1	9.1
Summer	6.7	6.7		None‡	None‡
2008
Winter	None‡	None‡		22.2	22.2
Summer	None‡	All§		None‡	None‡
2009
Winter	52.2	52.2		None‡	None‡
Summer**	22.2	22.2		None‡	None‡
2010
Winter	None‡	None‡		66.7	None‡
Summer	None‡	92.9		26.7	26.7
2011
Winter	87.5	87.5		64.3	64.3
Summer	None‡	None‡		37.5	37.5
2012
Winter	None‡	None‡		31.0	31.0
Summer	None‡	None‡		None‡	None‡

*Calendar months belonging to winter or summer influenza seasons were based on the average number of cases admitted per month during the study period (Figure 1). Winter seasons were November of previous year through April of current year; influenza usually peaks in February and March. Summer seasons were May–October; influenza usually peaks in July and August. HA, hemagglutinin.
†Co-circulation of multiple strains with a certain percentage closely matched the contemporary vaccine strain based on HA amino acid sequence comparison.
‡None of the circulating viruses examined were closely matched with the contemporary vaccine strain based on HA amino acid sequence comparison.
§All circulating viruses examined were closely matched with the contemporary vaccine strain based on HA amino acid sequence comparison.
¶A/Panama/1999, an alternate to A/Moscow/99, closely matched the circulating strains of 2001 winter and summer seasons and 2002 winter season.
#The World Health Organization recommended 2 A(H3N2) vaccine strains for the Northern Hemisphere in 2005, one of which, A/Wyoming/2003, closely matched a portion of the viruses circulating during the 2005 winter season.
**Dominated by influenza A(H1N1)pdm09.