Absence of influenza A(H1N1) during seasonal and pandemic seasons in a sentinel nursing home surveillance network in the Netherlands.

OBJECTIVES: To describe the epidemiological, virological, and institutional characteristics of influenza-like illness (ILI) in nursing homes (NHs). DESIGN: Continuous clinical surveillance of ILI and virological surveillance of ILI and other acute respiratory infections (ARIs) during four influenza seasons. SETTING: National sentinel NH surveillance network. PARTICIPANTS: National sentinel residents. MEASUREMENTS: Weekly registration of ILI cases (influenza seasons 2008/09-2009/10), influenza virus detection (influenza seasons 2006/07-2009/10), and collection of institutional characteristics of NHs at start of participation. RESULTS: During the 2008/09 influenza season, ILI incidence started to rise in Week 49 of 2008, peaked in Week 3 of 2009 (158 cases per 10,000 resident weeks), and flattened out by Week 16 of 2009 (mean ILI incidence during epidemic: 73 cases per 10,000 resident weeks). During the 2009/10 influenza pandemic, there was no epidemic peak. Influenza virus type and subtype varied throughout virological surveillance but was limited to influenza A(H3N2) and B viruses. Higher staff vaccination coverage (>15%) was associated with lower ILI-incidence in the 2008/09 influenza season in a univariate negative binomial regression analysis (incidence rate ratio = 0.3, 95% confidence interval = 0.1-0.8)). CONCLUSION: Neither seasonal nor pandemic influenza A(H1N1) viruses were detected in the network, despite widespread community transmission of seasonal and influenza A(H1N1) virus. ILI incidence trends corresponded to virological trends. Sentinel surveillance of ILI combining clinical and virological data in NHs increases understanding of transmission risks in this specific vulnerable population.