22 Million Cases of Swine Flu in U.S.

Nov. 12, 2009 -- H1N1 swine flu has killed more than 4,000 Americans -- perhaps as many as 6,000, the CDC now estimates.

Shockingly, 14 million to 34 million U.S. residents -- the CDC's best guess is 22 million -- came down with H1N1 swine flu by Oct. 17, the six-month anniversary of the beginning of the pandemic. There were about 98,000 hospitalizations (estimates range from 63,000 to 153,000).

In the four weeks since Oct. 17, H1N1 swine flu has been widespread across the nation. That means the new estimates, which greatly increase previous counts, will have risen sharply.

"We do think we are having a substantial number of deaths," CDC immunization and respiratory disease chief Anne Schuchat, MD, said at a news conference. "The numbers are only through Oct. 17, and we have seen a lot of deaths since then. Unfortunately, we will see more. ... I do believe the pediatric death toll will be extensive and much more than we have seen with seasonal flu."

How much the numbers have gone up will only be known when CDC epidemiologists are able to update the figures, which the CDC will do "every three or four weeks." However, it's becoming clear that a huge fraction of the population will have become ill before the flu season ends.

CDC data from Aug. 30 to Oct. 31 show flu activity is "substantially above historic levels in all U.S. surveillance systems," according to today's Morbidity and Mortality Weekly Report.

Nationwide, the number of flu patients showing up in doctors' offices and clinics was higher in September and October than at the peak of any flu season since record keeping began in 1997.

"We have tracked influenza for years. What we are seeing in 2009 is unprecedented," Schuchat said. "To have very high rates of flu in September and October is extremely unusual. ... If we look back, we don't see a fall like this."

Previous CDC estimates of H1N1 swine flu cases have been based on laboratory-confirmed infection. But not everyone who gets the flu is hospitalized with the flu, and not everyone who dies of the flu was tested. And the tests miss many people who actually do have flu.

To correct these underestimates, the CDC bases the new estimates on detailed clinical information reported by the Emerging Infections Network, a collaboration of 62 counties in 10 states, and on aggregate data reported from all states. This data is the used to derive estimates for the entire U.S.

"This is not a switch or a change, just a bigger picture," Schuchat said.

Young People Hit Hardest by H1N1 Swine Flu

During a normal flu season, nine out of 10 deaths are in elderly people. What makes the H1N1 swine flu death toll most shocking is that the vast majority of deaths have been in people under age 65 -- with a very large percentage under age 18.