CDC estimates of 2009 H1N1 influenza cases, hospitalizations and deaths in the United States, April 2009 – March 13, 2010
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CDC estimates of 2009 H1N1 influenza cases, hospitalizations and deaths in the United States, April 2009 – March 13, 2010

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      Estimating the number of individual flu cases in the United States is very challenging because many people with flu don’t seek medical care and only a small number of those that do seek care are tested. More people who are hospitalized or die of flu-related causes are tested and reported, but underreporting of hospitalizations and deaths occurs as well. For this reason CDC monitors influenza activity levels and trends and virus characteristics through a nationwide surveillance system and uses statistical modeling to estimate the burden of flu illness (including hospitalizations and deaths) in the United States.

      When the 2009 H1N1 flu outbreak began in April 2009, CDC began tracking and reporting the number of laboratory-confirmed 2009 H1N1 cases, hospitalizations and deaths as reported by states to CDC. These initial case counts (which were discontinued on July 24, 2009), and subsequent ongoing laboratory-confirmed reports of hospitalizations and deaths, are thought to represent a significant

      undercount of the actual number of 2009 H1N1 flu cases in the United States. A paper in Emerging Infectious Diseases authored by CDC staff entitled “Estimates of the Prevalence of Pandemic (H1N1) 2009, United States, April–July 2009” reported on a study to estimate the prevalence of 2009 H1N1 based on the number of laboratory-confirmed cases reported to CDC. Correcting for under-ascertainment, the study found that every case of 2009 H1N1 reported from April – July represented an estimated 79 total cases, and every hospitalized case reported may have represented an average of 2.7 total hospitalized people.

      CDC then began working on a way to estimate, in an ongoing way, the impact of

      the 2009 H1N1 pandemic on the U.S. in terms of 2009 H1N1 cases,

      hospitalizations and deaths. CDC developed a method to provide an estimated

      range of the total number of 2009 H1N1 cases, hospitalizations and deaths in the

      United States by age group using data on flu associated hospitalizations collected

      through CDC’s Emerging Infections Program.


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