CDC Novel H1N1 Flu | CDC Estimates of 2009 H1N1 Influenza Cases, Hospitalizations and Deaths in the United States

CDC Estimates of 2009 H1N1 Influenza Cases, Hospitalizations and Deaths in the United States, April – October 17, 2009
November 12, 2009, 1:00 PM ET

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 under-reporting 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 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. 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. 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. Since that time, CDC has been working to develop 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.

Method to Estimate 2009 H1N1 Cases, Hospitalizations and Deaths
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. This method uses data on influenza-associated hospitalizations collected through CDC’s Emerging Infections Program (EIP), which conducts surveillance for laboratory-confirmed influenza-related hospitalizations in children and adults in 62 counties covering 13 metropolitan areas of 10 states. To determine an estimated number of 2009 H1N1 hospitalizations nationwide, the EIP hospitalization data are extrapolated to the entire U.S. population and then corrected for factors that may result in under-reporting using a multiplier from “Estimates of the Prevalence of Pandemic (H1N1) 2009, United States, April–July 2009.”. The lower and upper hospitalization estimates also are calculated using the EIP hospitalization data. The national hospitalization estimates are then used to calculate deaths and cases. Deaths are calculated by using the proportion of laboratory-confirmed deaths to hospitalizations reported through CDC’s web-based Aggregate Hospitalization and Death Reporting Activity (AHDRA). Cases are estimated using multipliers derived from “Estimates of the Prevalence of Pandemic (H1N1) 2009, United States, April–July 2009.” The lower and upper end of the ranges for deaths and cases are derived from the lower and upper hospitalization estimates. The methods used to estimate impact may be modified as more information becomes available. More information about this methodology is available.
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. While EIP data is reported weekly during influenza season, because the system is based on reviews of patients medical charts there are sometimes delays in reporting and it can take some time for all the data to fill in. CDC will continue to provide weekly reports of influenza activity each Friday in FluView and will update the 2009 H1N1 Situation Update each Friday as well.

The estimated ranges of cases, hospitalizations and deaths generated by this method provide a sense of scale in terms of the burden of disease caused by 2009 H1N1. It may never be possible to validate the accuracy of these figures. The true number of cases, hospitalizations and deaths may lie within the range provided or it’s also possible that it may lie outside the range. The underlying assumption in this method is that the level of influenza activity (based on hospitalization rates) in EIP sites matches the level of influenza like illness (ILI) activity across the states.

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.

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CDC Novel H1N1 Flu | CDC Estimates of 2009 H1N1 Influenza Cases, Hospitalizations and Deaths in the United States