Between 1.8 million and 5.7 million Americans caught pandemic H1N1 influenza this spring, as many as 21,000 were hospitalized, and perhaps 800 died, according to new estimates by researchers at the Centers for Disease Control and Prevention. The revised numbers suggest that even larger numbers will become infected during this flu season.
Estimates, as opposed to specific numbers, are the best data available. Many cases are not reported to public health authorities, and the CDC stopped requiring laboratory confirmation of new cases when labs were becoming overwhelmed. Before now, officials were saying simply that more than a million Americans had been infected.
Epidemiologists from the CDC and the Harvard School of Public Health made the new estimates, which cover the beginning of the outbreak in April through July, in the CDC's online journal Emerging Infectious Diseases.
There were 43,677 laboratory-confirmed cases of what's called swine flu during the period, 5,009 hospitalizations and 302 deaths, but researchers think those are only the tip of the iceberg. Many infections were too mild for patients to seek medical care; many physicians didn't order tests for those who did; and many positive test results were not reported to health authorities, so the researchers had to use well-known models of disease spread to extrapolate from the cases that were reported. Similar techniques are used each year to estimate the prevalence of seasonal flu.
The team estimated that about 79 infections occurred for every one detected, giving a mean number of victims of about 3 million. The researchers also said that about 2.7 hospitalizations probably occurred for every one reported, giving a median estimate of 14,000. They did not directly estimate the number of deaths, but other data show that about 6% of those hospitalized died, leading to a figure of 800 deaths.
The researchers have not yet made similar calculations for the fall season, so CDC officials will say only that "many millions" of people have now been infected.
Meanwhile, officials said that an estimated 24.8 million doses of swine flu vaccine were available for shipment to the states Thursday, an increase of 1.6 million doses since the day before.
"We aren't where we want to be, but we are seeing forward progress," Dr. Anne Schuchat, director of the CDC's National Center for Immunization and Respiratory Diseases, said at a news conference. "Many doctor's offices are now beginning to get doses."
In other flu news:
* Doctors may have stumbled on a potential new therapy for the virus. Antiviral drugs such as Tamiflu, Relenza and now the intravenous drug peramivir have been the mainstays for treating swine flu, but a common, inexpensive and safe family of drugs may also be useful: statins. In addition to lowering cholesterol, statins reduce inflammation, which plays a role in development of the pneumonia that kills many hospitalized patients.
Dr. Ann Thomas of the Oregon Public Health Division reported Thursday at a Philadelphia meeting of the Infectious Diseases Society of America on a study of 2,800 people hospitalized with laboratory-confirmed seasonal flu in 10 states in 2007 to 2008; 801 of them were receiving statins, mostly people who were already on the drugs before they entered the hospital.
Thomas and her colleagues found that 3% of those not taking statins died but that only about 1.5% of those receiving the drugs did. Several researchers at the meeting called for testing to see if the drugs could help patients hospitalized with seasonal flu.
* Dutch scientists set off a small uproar Thursday with a "personal opinion" published in the journal Lancet Infectious Diseases arguing that catching seasonal flu could actually protect children against swine flu and other potentially related pandemic viruses.
Reaction was swift and highly negative. Other experts said there was no plausible biological explanation for such a finding.