CDC Announces New Method of More Accurately Calculating the Impact and Spread of H1N1

November 12, 2009

Case Counts, Deaths, and Hospitalizations will be Higher Based on New Model

CDC has announced a new method of calculating the impact of the H1N1 flu virus to get a more accurate picture of the spread of the disease. The new method provides estimates of the number of people infected, the number hospitalized, and the number of deaths in the U.S. in the first six months of the pandemic. The estimates are more complete than counts that only use laboratory-confirmed cases, since relying only on lab-confirmed cases results in an under-estimation of the virus' spread and impact.  The estimates do not reflect a change in disease incidence or severity, but what CDC believes is a more complete picture of the disease burden.

When the 2009 H1N1 flu outbreak began in April 2009, CDC began reporting the number of laboratory-confirmed cases, hospitalizations and deaths associated with 2009 H1N1 flu in the United States that were reported by states to CDC.  As CDC has frequently noted, these initial case counts, 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.

For this reason, CDC has developed a method to provide an estimated range of the total number of 2009 H1N1 cases, hospitalizations and deaths in the United States since April, 2009, as well as a breakdown of these estimates by age groups. It’s important to note that these figures are estimates.

These estimates are based on data on 2009 H1N1 hospitalizations collected through CDC’s Emerging Infections Program (EIP), which conducts surveillance for laboratory-confirmed influenza-related hospitalizations in children and adults. The data is then corrected to account for under-reporting of flu illness.

Throughout the remainder of the 2009 H1N1 pandemic CDC will update the range of estimated 2009 H1N1 cases, hospitalizations and deaths every three or four weeks. This methodology is not a predictive tool and cannot be used to forecast the number of cases, hospitalizations and deaths that will occur going forward over the course of the pandemic because they are based on actual surveillance data.

Estimates:

• CDC estimates that between 14 million and 34 million cases of 2009 H1N1 occurred between April and October 17, 2009. The mid-level in this range is about 22 million people infected with 2009 H1N1.
• CDC estimates that between about 63,000 and 153,000 2009 H1N1-related hospitalizations occurred between April and October 17, 2009. The mid-level in this range is about 98,000 H1N1-related hospitalizations.
• CDC estimates that between about 2,500 and 6,000 2009 H1N1-related deaths occurred between April and October 17, 2009. The mid-level in this range is about 3,900 2009 H1N1-related deaths.

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