Millions of Americans Infected With Pandemic Flu

From 1.8 to 5.7 million Americans experienced symptomatic swine flu (2009 H1N1 pandemic influenza) between April and July 23rd of this year. This estimate is based on a "probabilistic multiplier model" that adjusts for the underascertainment of pandemic flu cases in the United States. The results of the model, which was created and applied by investigators at the CDC and the Harvard School of Public Health, are available in an expedited article in the journal Emerging and Infectious Diseases.

The probabilistic multiplier model was used to adjust for the underascertainment of pandemic flu cases at each of the following steps (as diagrammed in the provided figures): the pursuit of medical care; the collection of specimens from persons seeking medical care; the submission of specimens for confirmatory testing (ie, RT-PCR); the laboratory detection of the 2009 H1N1 virus; and the reporting of confirmed cases. The model was adjusted separately for hospitalized patients (B)—who, by definition, had already sought medical care.

On the basis of the model, investigators conclude the following for the time period between April and July of this year:

• Every reported case of pandemic flu represents 79 total cases (90% probability range, 47-148).
• The estimated median number of symptomatic cases is 3.0 million.
• The estimated incidence of pandemic flu in persons ≥65 years of age is 107/100,000.
• The estimated incidence of pandemic flu in persons 5-24 years of age is 2196/100,000.
• Every hospitalized case of pandemic flu represents 2.7 total hospitalized cases (90% range, 1.9-4.3)
• The estimated median number of hospitalizations is 14,000 (range, 9000-21,000).
• The estimated ratio of hospitalizations to total symptomatic cases is 0.45% (90% range, 0.16%-1.2%).
• The estimated median incidence of hospitalizations in persons <5 years of age is 13.0/100,000.
• The ratio of deaths to hospitalizations was 6%.* (Note: This value was not derived from the model).
• The estimated median number of deaths is therefore 800 (90% range, 550-1300).

"Because this [death] assumption has several limitations," the authors conclude, "more sophisticated models are also being developed to better understand the severity of the US epidemic in the spring of 2009, including intensive care unit admissions and deaths."

RT-PCR = reverse transcriptase polymerase chain reaction.

* Therefore the overall estimated death rate (although this value is not provided in the article) is 0.027%—which is within the range provided by one of the authors, Harvard epidemiologist Marc Lipsitch, to Reuters in September.

Update: From the AP by way of the CDC—At least 114 American children have now died of complications due to the 2009 H1N1 virus.