CDC Estimates of 2009 H1N1 Influenza Cases, Hospitalizations and Deaths in the United States, April – November 14, 2009

December 10, 2009, 1:00 PM ET
Excerpt:

The Numbers

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On Novebmer 12, 2009 CDC provided the first set of estimates on the numbers of 2009 H1N1 cases and related hospitalizations and deaths in the United States between April and October 17, 2009.

Estimates from April – October 17, 2009:

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

Updated Estimates from April – November 14, 2009

Using the same methodology CDC has updated the estimates to include the time period from April through November 14, 2009.

• CDC estimates that between 34 million and 67 million cases of 2009 H1N1 occurred between April and November 14, 2009. The mid-level in this range is about 47 million people infected with 2009 H1N1.
• CDC estimates that between about 154,000 and 303,000 2009 H1N1-related hospitalizations occurred between April and November 14, 2009. The mid-level in this range is about 213,000 H1N1-related hospitalizations.
• CDC estimates that between about 7,070 and 13,930 2009 H1N1-related deaths occurred between April and November 14, 2009. The mid-level in this range is about 9,820 2009 H1N1-related deaths.

Note: More than 95% of the increases in the estimated numbers of 2009 H1N1 cases, hospitalizations and deaths between the November 12 and December 10 estimates occurred between October 17 and November 14, 2009. (Less than 5% of increases are the result of delayed reporting in cases, hospitalizations and deaths that occurred prior to October 17, 2009.)

CDC Estimates of 2009 H1N1 Cases and Related Hospitalizations and Deaths from April-November 14, 2009, By Age Group

* Deaths have been rounded to the nearest ten. Hospitalizations have been rounded to the nearest thousand and cases have been rounded to the nearest million. Exact numbers also are available.

The results of this method confirm previous epidemiological data indicating that this disease primarily affects people younger than 65 year old, with the number of cases, hospitalizations and deaths overwhelmingly occurring in people 64 years and younger. This is very different from seasonal influenza, where about 60 percent of seasonal flu-related hospitalizations and 90 percent of flu-related deaths occur in people 65 years and older. The proportion of younger people being impacted by 2009 H1N1 is much greater than what occurs during seasonal flu and people 65 and older are much less affected by this virus than what routinely occurs with seasonal influenza. The results generated by this method also underscore the continued importance of the 2009 H1N1 vaccination program and support the recommended target groups for vaccination.

This methodology and the resulting estimates also underscore the substantial under-reporting that occurs when laboratory-confirmed outcomes are the sole method used to capture hospitalizations and deaths. Since the outbreak began in April, states have reported 2009 H1N1 hospitalizations and deaths to CDC. Cumulative reports of laboratory-confirmed 2009 H1N1 hospitalizations and deaths for the same period used in this analysis (April through October 17, 2009), are 17,283 hospitalizations and 1,004 deaths. CDC has maintained since the beginning of this outbreak that laboratory-confirmed data on hospitalizations and deaths reported to CDC is an underestimation of the true number that have occurred because of incomplete testing, inaccurate test results, or diagnosis that attribute hospitalizations and deaths to other causes, for example, secondary complications to influenza. (Information about surveillance and reporting for 2009 H1N1 is available at Questions and Answers: Monitoring Influenza Activity, Including 2009 H1N1.)

The estimates derived from this methodology provide the public, public health officials and policy makers a sense of the health impact of the 2009 H1N1 pandemic. While these numbers are an estimate, CDC feels that they present a fuller picture of the burden of 2009 H1N1 disease on the United States.

CDC will continue to use weekly data from systems that comprise the National Influenza Surveillance System to monitor geographic, temporal and virologic trends in influenza in the nation.