TABLE 1.

Prevalence of antibodies against different influenza A viruses in sera from unvaccinated people in Jiangsu, China

Subtype	Virus strain	Positive number at different HI (rate, 95% CI) (n=1039)
		HI≥10	HI≥20	HI≥40	HI≥80
H1N1	Brisbane/59/2007	126 (12.1%,10.2-14.3)	51 (4.9%,3.7-6.4)	14 (1.4%,0.7-2.2)	4 (0.4%,0.1-1.0%)
panH1N1	California/07/2009	88 (8.5%,6.9-10.3)	57 (5.5%,4.2-7.0)	32 (3.1%,2.1-4.3)	11 (1.1%,0.5-1.9)
H3N2	Perth/16/2009-like	435 (41.9%,38.8-44.9)	335 (32.2%,29.4-35.2)	174 (16.8%,14.5-19.2)	69 (6.6%,5.2-8.3)
H5N1	Shenzhen/406H/2006(Clade 2.3.4)	27 (2.6%,1.7-3.8)	3 (0.3%,0.1-0.8)	0 (0.0%,0.0-0.4)	0 (0.0%,0.0-0.4)
	BGs/Qinghai/15C/2005(Clade 2.2)	3 (0.3%,0.1-0.8)	2 (0.2%,0.0-0.7)	0 (0.0%,0.0-0.4)	0 (0.0%,0.0-0.4)
H9N2	A/HK/2103/03	9 (0.9%,0.4-1.6)	1 (0.1%,0.1-0.5)	0(0.0%,0.0-0.4)	0 (0.0%,0.0-0.4)
	A//HK//464419/09	149 (14.3%,12.3-16.6)	62 (6.0%,4.6-7.6)	15(1.4%,0.8-2.4)	3 (0.3%,0.1-0.8)
H6N1	A/ duck/ JX227/03	0 (0.0%,0.0-0.4)	0 (0.0%,0.0-0.4)	0 (0.0%,0.0-0.4)	0 (0.0%,0.0-0.4)

1039 sera samples with approximately 200 samples in each of five birth year groups five birth year groups (1942-1951, 1952-1961, 1962-1971, 1972-1981, 1982-1991) were collected from residents of two villages in Jiangsu Province, China in May 2010. Antibodies to reference strains of influenza A virus were examined by hemagglutination inhibition assay (9). For H5N1 virus, HI assays were also conducted using two strains belonging to clade 7: no positive reactions were detected. For H9N2, two strains representing the G1 and CK/Bei lineages were used and all positive sera with HI ≥40 were tested against other major influenza virus subtypes to exclude cross-reactivity. Neutralization assays were also performed to confirm the presence of specific antibodies to H9 that did not cross-react with other subtypes of influenza virus. Specimens were tested in duplicate. Estimation of the 95% CI was performed with exact binomial methods. BGs, bar-headed goose; A/HK/2103/03 (CK/Bei-like); A/HK/464419/09 (G1-like).