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Questions and Answers: EID article "Estimates of the Prevalence of Pandemic (H1N1) 2009, United States, April-July 2009"

October 28, 2009, 4:30 PM ET


Through July 2009, a total of 43,677 laboratory-confirmed cases of 2009 H1N1 were reported in the United States, which is likely a substantial underestimate of the true number. Correcting for under-ascertainment using a multiplier model, researchers in this study estimate there may have been between 1.8 million and 5.7 million cases during this time period, including 9,000-21,000 hospitalizations. This article is available online at Emerging Infectious Diseases online.

Questions & Answers

What was the main purpose for conducting this study?

It is likely that the 43,677 laboratory-confirmed cases of 2009 H1N1 reported between April and July of 2009 are a substantial underestimation of the true number of cases for this time period.  The current study, which used a relatively quick and simple approach, was conducted to help estimate the true number of cases, and the human health impact of 2009 H1N1 during the first four months of the pandemic.

How was this study conducted?

To estimate the total number of cases from April-July 2009, researchers built a probabilistic multiplier model that adjusts the count of laboratory-confirmed cases for each of the following steps:  medical care seeking, specimen collection, submission of specimens for confirmation, laboratory detection of 2009 H1N1, and reporting of confirmed cases.

This statistical model was based on a widely accepted technique that has been used previously to estimate the actual number of cases of food-borne illness. 1This model collected information on the number of 2009 H1N1 laboratory-confirmed cases, hospitalizations and deaths in the United States reported to CDC from April – July 2009. These numbers were then adjusted using multipliers to correct for factors that can lead to under-counting. These multipliers and adjustments were made based on analysis of community surveys, outbreak investigations and published data.

What did the study’s findings indicate?

Using the model, researchers estimate that from April-July 2009 the number of people infected with 2009 H1N1 may have been up to 140 times greater than the reported number of laboratory confirmed cases. They estimate that between 1.8 million and 5.7 million cases, including 9,000 – 21,000 hospitalizations, may have occurred during the time period. This indicates that every case of 2009 H1N1 reported from April – July represents an estimated 79 total cases, and every hospitalized case reported may represent a median of 2.7 total hospitalized persons.

  1. Mead PS et al. “Food-related illness and death in the United States.” Emerg Infect Dis. 1999 Sep-Oct; 5(5):607-23.
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