Discordant Correlation between Serological Assays Observed When Measuring Heterosubtypic Responses against Avian Influenza H5 and H7 Viruses in Unexposed Individuals

Table 2

Panel of 68 sera collected from Italian population between 1992 and 2007 and tested against H1, H5, H3, and H7 influenza pseudotypes. Number of positive sera was scored based on different positive thresholds (also expressed as a percentage).

	A/Vietnam/1194/04; H5N1	A/Indonesia/5/05; H5N1	A/ck/Italy/13474/99; H7N1	A/Udorn/307/72; H3N2	A/South Carolina/1/18; H1N1
1 : 40; cutoff*	61/68; (88%)	42/68; (62%)	22/68; (32%)	37/37; (100%)	37/37; (100%)
1 : 80; cutoff**	56/68; 82.3%	30/68; 44.1%	12/68; 17.6%	37/37; 100%	34/37; 92%
1 : 357; cutoff***	36/68; 53%	14/68; 20.5%	All negative	37/37; 100%	25/37; 67.5%
≥20; cutoff****	67/68; (98.5%)	44/68; (65%)	53/68; (78%)	37/37; 100%	37/37; 100%

∗HI positive cut-off which defines sero-protection in adults for seasonal vaccines [25].
∗∗Proposed positive threshold for MN assay used to test human sera against HPAI strains [41].
∗∗∗Corresponds for pp-NT assay to the proposed threshold of 1 : 80 for MN assay as previously shown [30].
∗∗∗∗Positive cut-off suggested for the study of hetero-subtypic antibody responses by using influenza pseudotyped particles [14].