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From the Centers for Disease Control and Prevention
January 25, 2012

Notice to Readers: Revised Estimates of the Public Health Impact of 2009 Pandemic Influenza A (H1N1) Vaccination

JAMA. 2012;307(4):358. doi:

In the May 20, 2011, report, “Ten Great Public Health Achievements—United States, 2001-2010,” on page 621, preliminary estimates of the impact of public health interventions during the 2009 H1N1 pandemic were presented as follows: “These public health interventions prevented an estimated 5-10 million cases, 30,000 hospitalizations, and 1,500 deaths.1” These estimates were derived using combined data from two sources: (1) an unpublished CDC model for estimating the impact of the 2009 H1N1 pandemic influenza vaccine on averting cases, hospitalizations, and deaths during the 2009-10 influenza season and (2) a model for estimating the impact of antiviral treatment in averting hospitalizations and deaths during the 2009-10 season.2 As a result of a programming error, the model used to estimate the impact of vaccination did not adequately adjust for the decreasing risk for disease as the pandemic progressed, and thus the impact of vaccination was overestimated.

The corrected estimates for the combined impact from vaccine and antiviral treatment are as follows: 713,000 to 1.5 million cases, 12,300 to 23,000 hospitalizations, and 620 to 1,160 deaths averted. Of these, 713,000 to 1.5 million cases, 3,900 to 10,400 hospitalizations, and 200 to 520 deaths were averted as a result of the vaccination campaign (CDC, unpublished data, 2011), whereas the use of influenza antiviral medications is estimated to have prevented another 8,400 to 12,600 hospitalizations and another 420 to 640 deaths.2

It is important to note that the error does not involve nor pertain to the effectiveness of the 2009 H1N1 vaccine, nor to estimates of the burden of the 2009 H1N1 pandemic, which resulted in approximately 43 million to 89 million cases, 195,000 to 403,000 hospitalizations, and 8,900 to 18,300 deaths, including 910 to 1,880 deaths among children aged <18 years, during April 2009–April 2010.3 CDC-supported evaluations have shown that the vaccine was effective in preventing influenza medical visits during the pandemic.4 However, because there was early widespread circulation of the 2009 H1N1 virus, many persons in the United States became ill before vaccine was available.

CDC continues to work on developing and evaluating statistical models for estimating the impact of influenza vaccination in order to develop better programs and ways to monitor the impact of those programs. CDC expects this work might lead to future publications that provide additional impact estimates.

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