Deconstructing CDC estmates of H1N1 influenza deaths and hospitalizations

In a widely reported story, the Center for Disease Control (CDC) presented revised estimates on H1N1 mortality, including 540 estimated deaths among children to date; 35 such deaths were reported for the week ending 7 November.

  H1N1 Seasonal
Deaths 3,900 36,000
Pediatric deaths 540 80
Hospitalizations 98,000 200,000

Source: CDC

This raises questions as to how CDC arrives at these numbers. The task is complicated by the fact that many people with flu don’t seek medical care; few who do are tested. Hospitalizations and deaths are also underreported.

To correct for this, CDC applies a median “multiplier” of 2.7 to reported and extrapolated hospitalizations for influenza like illnesses (ILI) obtained from the Emerging Infections Program (EIP), a surveillance network spanning ten states.

This multiplier is derived from Monte Carlo simulations on a model that adjusts for “underrecognized” factors and health-seeking behaviors. Example: if the percentage of persons seeking medical care increases, multiplier effects would fall.

Deaths are computed using a fraction (around four percent overall) of estimated hospitalizations. However, these percentages also vary across age groups.

CDC: 2009 H1N1-Related Deaths, Hospitalizations and Cases: Details of Extrapolations and Ranges: U.S., Emerging Infections Program (EIP) Data – PDF
CDC: Estimates of 2009 H1N1 Influenza Cases, Hospitalizations and Deaths in the United States, April – October 17, 2009
CDC: Estimates of the Prevalence of Pandemic (H1N1) 2009, United States, April–July 2009 – PDF
USA Today: Swine flu has killed 540 kids, sickened 22 million Americans

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