Influenza A(H1N1) Oseltamivir Resistant Viruses in the Netherlands During the Winter 2007/2008

Table 1.: Results of Phenotypic (Amount of NAI Resistance Measured by IC₅₀-Value for Oseltamivir) and Genotypic Analysis (Presence of Substitution H275Y in the NA) to Determine Oseltamivir Resistance of Influenza A(H1N1) Viruses

Source of Influenza Viruses A(H1N1)	Phenotypic Analysis Only	Genotypic Analysis Only	Phenotypic and Genotypic Analysis	Phenotypic Oseltamivir Susceptibility	Combined Analysis
	Increased IC₅₀-values^a, n/N (%)	H275Y in NA, n/N (%)	Increased IC₅₀-values^a and H275Y in NA, n/N (%)	Mean (and range) of IC_50-value of viruses with increased IC₅₀-values, ^a nM	Oseltamivir resistance^b, n/N (%)
Sentinel	4/10 (40%)	5/33 (15%)	5/28 (18%)^c	362,5 (282 – 538)^d	14/71 (19.7%)
Non-sentinel		2/4 (50%)	30/96 (31%)	470 (290 - 828)	32/100 (32%)
Overall (sentinel and non-sentinel)	4/10 (40%)	7/37 (19%)	35/124 (18%)	448 (282 – 828)	46/171 (27%)

^a >100-fold compared to NAI sensitive A(H1N1) viruses (mean±SD IC₅₀-value: 1.21±0.41nM).

^b With >100-fold increased IC₅₀-values OR H275Y in NA.

^c 1 mixed population included (wild type, H275Y).

^d 1 mixed population excluded.

NAI = neuraminidase inhibitors; NA = neuraminidase; IC₅₀ = 50% inhibitory concentration.