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, virologi-cal, and institutional characteristics of influenza-like illness(ILI) in nursing homes (NHs).DESIGN: Continuous clinical surveillance of ILI and viro-logical surveillance of ILI and other acute respiratoryinfections (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 virusdetection (influenza seasons 2006/07–2009/10), and collec-tion of institutional characteristics of NHs at start ofparticipation.RESULTS: During the 2008/09 influenza season, ILI inci-dence started to rise in Week 49 of 2008, peaked in Week3 of 2009 (158 cases per 10,000 resident weeks), and flat-tened out by Week 16 of 2009 (mean ILI incidence duringepidemic: 73 cases per 10,000 resident weeks). During the2009/10 influenza pandemic, there was no epidemic peak.Influenza virus type and subtype varied throughout viro-logical surveillance but was limited to influenza A(H3N2)and B viruses. Higher staff vaccination coverage (>15%)was associated with lower ILI-incidence in the 2008/09influenza season in a univariate negative binomial regres-sion analysis (incidence rate ratio = 0.3, 95% confidenceinterval = 0.1–0.8)).CONCLUSION: Neither seasonal nor pandemic influenzaA(H1N1) viruses were detected in the network, despitewidespread community transmission of seasonal and influ-enza A(H1N1) virus. ILI incidence trends corresponded tovirological trends. Sentinel surveillance of ILI combiningclinical and virological data in NHs increases understand-ing of transmission risks in this specific vulnerablepopulation. J Am Geriatr Soc 59:2301–2305, 2011.