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Cold, Flu, & Cough Health Center

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140 Times More Early Swine Flu Than Reported

CDC: By August, Up to 5.7 Milllion Americans Had H1N1 Swine Flu
WebMD Health News
Reviewed by Louise Chang, MD

Oct. 29, 2009 - By the end of July, up to 5.7 million Americans -- 140 times the reported number -- had H1N1 swine flu.

As many as 21,000 were hospitalized by July 23, the CDC now estimates. This suggests that the current number of H1N1 swine flu-related hospitalizations -- just under 22,000 from the end of August to the middle of October -- is a vast underestimate.

When reporting numbers of hospitalizations and deaths, CDC officials have always noted that the data are a guide to the severity of the pandemic and not a precise tally.

There actually were 79 H1N1 swine flu cases for every lab-confirmed case and about three pandemic flu-related hospitalizations for every reported hospitalization through July 23, according to the new estimates from CDC epidemiologists Carrie Reed, PhD; Lyn Finelli, DrPH; and colleagues.

After July 23, the burgeoning number of flu cases made it necessary to stop counting lab-confirmed cases -- which the CDC always warned was " just the tip of the iceberg" -- and to start using mathematical models to track the pandemic.

"We don't have an update since July 23 with this modelling approach, but as [CDC Director Thomas] Frieden said last week, we do believe many millions of people have already contracted this virus here in the U.S.," CDC respiratory disease chief Anne Schuchat, MD, said today at a news conference.

The key question -- how many millions of Americans have come down with H1N1 swine flu -- remains unanswered. But there's been more than a fivefold increase in cumulative flu-related hospitalizations and deaths since Aug. 30.

Schuchat warned against applying the multipliers used in the Reed/Finelli model to current data.

"We don't actually even get reports any more of the individual cases, we only get summary reports of hospitalizations, so the case multiplier would be a challenging thing to track," Schuchat said. "The hospitalization multiplier might also have to be taken with a grain of salt at this time, because of the possibility that some people were being hospitalized early in this outbreak for different reasons than they would be now."

Reed, Finelli, and colleagues report their findings in an early ahead-of-print release from the CDC online journal Emerging Infectious Diseases.

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