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Influenza Illness and Hospitalizations Averted by Influenza Vaccination in the United States, 2005–2011

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      Abstract

      ContextThe goal of influenza vaccination programs is to reduce influenza-associated disease outcomes. Therefore, estimating the reduced burden of influenza as a result of vaccination over time and by age group would allow for a clear understanding of the value of influenza vaccines in the US, and of areas where improvements could lead to greatest benefits.ObjectiveTo estimate the direct effect of influenza vaccination in the US in terms of averted number of cases, medically-attended cases, and hospitalizations over six recent influenza seasons.DesignUsing existing surveillance data, we present a method for assessing the impact of influenza vaccination where impact is defined as either the number of averted outcomes or as the prevented disease fraction (the number of cases estimated to have been averted relative to the number of cases that would have occurred in the absence of vaccination).ResultsWe estimated that during our 6-year study period, the number of influenza illnesses averted by vaccination ranged from a low of approximately 1.1 million (95% confidence interval (CI) 0.6–1.7 million) during the 2006–2007 season to a high of 5 million (CI 2.9–8.6 million) during the 2010–2011 season while the number of averted hospitalizations ranged from a low of 7,700 (CI 3,700–14,100) in 2009–2010 to a high of 40,400 (CI 20,800–73,000) in 2010–2011. Prevented fractions varied across age groups and over time. The highest prevented fraction in the study period was observed in 2010–2011, reflecting the post-pandemic expansion of vaccination coverage.ConclusionsInfluenza vaccination programs in the US produce a substantial health benefit in terms of averted cases, clinic visits and hospitalizations. Our results underscore the potential for additional disease prevention through increased vaccination coverage, particularly among nonelderly adults, and increased vaccine effectiveness, particularly among the elderly.

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      Most cited references 29

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      Influenza-associated hospitalizations in the United States.

      Respiratory viral infections are responsible for a large number of hospitalizations in the United States each year. To estimate annual influenza-associated hospitalizations in the United States by hospital discharge category, discharge type, and age group. National Hospital Discharge Survey (NHDS) data and World Health Organization Collaborating Laboratories influenza surveillance data were used to estimate annual average numbers of hospitalizations associated with the circulation of influenza viruses from the 1979-1980 through the 2000-2001 seasons in the United States using age-specific Poisson regression models. We estimated influenza-associated hospitalizations for primary and any listed pneumonia and influenza and respiratory and circulatory hospitalizations. Annual averages of 94,735 (range, 18,908-193,561) primary and 133,900 (range, 30,757-271,529) any listed pneumonia and influenza hospitalizations were associated with influenza virus infections. Annual averages of 226,54 (range, 54,523-430,960) primary and 294,128 (range, 86,494-544,909) any listed respiratory and circulatory hospitalizations were associated with influenza virus infections. Persons 85 years or older had the highest rates of influenza-associated primary respiratory and circulatory hospitalizations (1194.9 per 100,000 persons). Children younger than 5 years (107.9 primary respiratory and circulatory hospitalizations per 100,000 persons) had rates similar to persons aged 50 through 64 years. Estimated rates of influenza-associated hospitalizations were highest during seasons in which A(H3N2) viruses predominated, followed by B and A(H1N1) seasons. After adjusting for the length of each influenza season, influenza-associated primary pneumonia and influenza hospitalizations increased over time among the elderly. There were no significant increases in influenza-associated primary respiratory and circulatory hospitalizations after adjusting for the length of the influenza season. Significant numbers of influenza-associated hospitalizations in the United States occur among the elderly, and the numbers of these hospitalizations have increased substantially over the last 2 decades due in part to the aging of the population. Children younger than 5 years had rates of influenza-associated hospitalizations similar to those among individuals aged 50 through 64 years. These findings highlight the need for improved influenza prevention efforts for both young and older US residents.
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        Estimates of the Prevalence of Pandemic (H1N1) 2009, United States, April–July 2009

        Through July 2009, a total of 43,677 laboratory-confirmed cases of influenza A pandemic (H1N1) 2009 were reported in the United States, which is likely a substantial underestimate of the true number. Correcting for under-ascertainment using a multiplier model, we estimate that 1.8 million–5.7 million cases occurred, including 9,000–21,000 hospitalizations.
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          Effectiveness of inactivated influenza vaccines varied substantially with antigenic match from the 2004-2005 season to the 2006-2007 season.

          We estimated the effectiveness of inactivated influenza vaccines for the prevention of laboratory-confirmed, medically attended influenza during 3 seasons with variable antigenic match between vaccine and patient strains. Patients were enrolled during or after a clinical encounter for acute respiratory illness. Influenza infection was confirmed by culture or reverse-transcriptase polymerase chain reaction. Case-control analyses were performed that used data from patients who were ill without influenza (hereafter, "test-negative control subjects") and data from asymptomatic control subjects from the population (hereafter, "traditional control subjects"). Vaccine effectiveness (VE) was estimated as [100 x (1-adjusted odds ratio)]. Influenza isolates were antigenically characterized. Influenza was detected in 167 (20%) of 818 patients in 2004-2005, in 51 (14%) of 356 in 2005-2006, and in 102 (11%) of 932 in 2006-2007. Analyses that used data from test-negative control subjects showed that VE was 10% (95% confidence interval [CI], -36% to 40%) in 2004-2005, 21% (95% CI, -52% to 59%) in 2005-2006, and 52% (95% CI, 22% to 70%) in 2006-2007. Using data from traditional control subjects, VE for those seasons was estimated to be 5% (95% CI, -52% to 40%), 11% (95% CI, -96% to 59%), and 37% (95% CI, -10% to 64%), respectively; confidence intervals included 0. The percentage of viruses that were antigenically matched to vaccine strains was 5% (3 of 62) in 2004-2005, 5% (2 of 42) in 2005-2006, and 91% (85 of 93) in 2006-2007. Influenza VE varied substantially across 3 seasons and was highest when antigenic match was optimal. VE estimates that used data from test-negative control subjects were consistently higher than those that used data from traditional control subjects.
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            Author and article information

            Affiliations
            [1 ]Influenza Division, National Center for Immunizations and Respiratory Diseases, Centers for Disease Control and Prevention, Atlanta, Georgia, United States of America
            [2 ]Immunizations Services Division, Centers for Disease Control and Prevention, Atlanta, Georgia, United States of America
            [3 ]Division of Preparedness and Emerging Infections, National Center for Enteric and Zoonotic Infectious Disease, Centers for Disease Control and Prevention, Atlanta, Georgia, United States of America
            Harvard School of Public Health, United States of America
            Author notes

            Competing Interests: The authors have declared that no competing interests exist.

            Contributed reagents/materials/analysis tools: PMG DKS JAS PJL. Wrote the paper: DK. Reveiwed the manuscript: DK CR LF PYC PMG DKS JAS MIM PJL JSB. Edited the manuscript: DK CR LF PYC PMG DKS JAS MIM PJL JSB. Conceptualized the analysis: DK CR. Conceived the paper: LF JSB.

            Contributors
            Role: Editor
            Journal
            PLoS One
            PLoS ONE
            plos
            plosone
            PLoS ONE
            Public Library of Science (San Francisco, USA )
            1932-6203
            2013
            19 June 2013
            : 8
            : 6
            23840439
            3686813
            PONE-D-13-03355
            10.1371/journal.pone.0066312
            (Editor)

            This is an open-access article, free of all copyright, and may be freely reproduced, distributed, transmitted, modified, built upon, or otherwise used by anyone for any lawful purpose. The work is made available under the Creative Commons CC0 public domain dedication.

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            Pages: 8
            Funding
            The authors have no support or funding to report.
            Categories
            Research Article
            Biology
            Computational Biology
            Population Modeling
            Infectious Disease Modeling
            Population Biology
            Epidemiology
            Infectious Disease Epidemiology
            Medicine
            Epidemiology
            Infectious Disease Epidemiology
            Infectious Diseases
            Viral Diseases
            Influenza
            Infectious Disease Control
            Infectious Disease Modeling
            Pulmonology
            Respiratory Infections
            Upper Respiratory Tract Infections

            Uncategorized

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