Clinical Characteristics and 30-Day Outcomes for Influenza A 2009 (H1N1), 2008-2009 (H1N1), and 2007-2008 (H3N2) Infections

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Abstract
Context The clinical characteristics of pandemic 2009 influenza A(H1N1) infections have not been compared directly with illnesses caused by other influenza A strains. Objective To compare clinical features and outcomes for 2009 H1N1, seasonal H1N1, and H3N2 influenza in a population-based cohort. Design, Setting, and Participants Active surveillance with 30-day follow-up for influenza cases among children and adults living in a 14–zip code area in Wisconsin. Patients with subjective fever, chills, or cough of fewer than 8 days' duration were screened for eligibility during an outpatient or inpatient encounter. Consenting patients were interviewed and tested for influenza A during the 2007-2008 and 2008-2009 influenza seasons and from May to November 2009; 6874 patients (70%-86% of eligible patients) agreed to participate. Medical records were reviewed to assess outcomes. Main Outcome Measures Hospital admission, radiographically confirmed pneumonia, and clinical characteristics of influenza A by strain. Results We identified 545 2009 H1N1, 221 seasonal H1N1, and 632 H3N2 infections. The median ages of infected participants were 10, 11, and 25 years, respectively (P < .001). Hospital admission occurred within 30 days for 6 of 395 children with 2009 H1N1 (1.5%; 95% confidence interval [CI], 0.6%-3.1%), 5 of 135 with seasonal H1N1 (3.7%; 95% CI, 1.4%-8.0%), and 8 of 255 with H3N2 (3.1%; 95% CI, 1.5%-5.9%). Among adults, hospital admission occurred in 6 of 150 with 2009 H1N1 (4.0%; 95% CI, 1.6%-8.1%), 2 of 86 with seasonal H1N1 (2.3%; 95% CI, 0.3%-8.1%), and 17 of 377 with H3N2 (4.5%; 95% CI, 2.7%-7.0%). Pneumonia occurred in 10 children with 2009 H1N1 (2.5%; 95% CI, 1.3%-4.5%), 2 with seasonal H1N1 (1.5%; 95% CI, 0.2%-5.2%), and 5 with H3N2 (2.0%; 95% CI, 0.7%-4.3%). Among adults, pneumonia occurred in 6 with 2009 H1N1 (4.0%; 95% CI, 1.6%-8.1%), 2 with seasonal H1N1 (2.3%; 95% CI, 0.3%-8.1%), and 4 with H3N2 (1.1%; 95% CI, 0.3%-2.7%). Conclusions In this population, individuals with 2009 H1N1 infection were younger than those with H3N2. The risk of most serious complications was not elevated in adults or children with 2009 H1N1 compared with recent seasonal strains.