Hundreds of thousands of Americans could die over the next two years if the vaccine and other control measures for the new H1N1 influenza are not effective, and, at the pandemic's peak, as much as 40% of the workforce could be affected, according to new estimates from the Centers for Disease Control and Prevention.
That is admittedly a worst-case scenario that the federal agency says it doesn't expect to occur.
But the broad range of potential deaths highlights the unpredictability of flu viruses in general and this swine flu virus in particular because it hasn't behaved the way researchers have expected.
The number of potential deaths is much higher than that usually seen in seasonal flu, which kills an estimated 36,000 Americans a year, and is even higher than the nation's most recent pandemic.
A 2003 CDC study of deaths in the preceding decade showed yearly totals ranging from 17,000 to 52,000. The 1957 pandemic of Asian flu killed 70,000.
The vaccine the CDC is counting on is not expected to be available before the end of September, too late to immunize children before the start of school, where the proximity of children is expected to speed up transmission of the virus.
But testing of the vaccine has already begun at some sites, and authorities have no reason to believe it will fail because the technology for flu vaccines has been thoroughly demonstrated.
Rather, the questions have been about how long it will take to produce a new vaccine and how many doses will be available.
So far, the CDC has given no estimate of how many deaths might occur if the vaccine works.
The new estimates, prepared more than a month ago but released only Friday to the Associated Press, are not based on an enhanced lethality of the new H1N1 virus, but rather on the lack of resistance to the virus in the general population and its continuing spread through the summer months, when flu viruses normally are less active.
Twenty states are now reporting widespread or regional flu activity, with the H1N1 virus predominating, according to Dr. Anne Schuchat, director of the CDC's National Center for Immunization and Respiratory Diseases. "That's very unusual at this time of year," she said. "It's a testament to how susceptible people are to the virus."
Cases this summer have been concentrated in camps and military installations where young people are gathered. "This has been a challenging summer for camps, with a lot of outbreaks" and closures, Schuchat said.
There have been many media reports of summer camps urging children to bring the antiviral drug Tamiflu and take it prophylactically, and that has concerned the CDC because such widespread use could lead to increased resistance to the drug, which is the first line of defense in treating infected people.
In cases the agency has investigated, however, it has been unable to confirm such reports.
The agency recommends that the drug be administered only to infected patients and as a preventive measure to high-risk people who are in close contact with flu patients.
To date, only five cases of Tamiflu-resistant viruses have been observed worldwide, and there is no evidence that such viruses have been spread from person to person, she said. But virtually all the strains of seasonal flu that are now circulating are resistant to the drug and the genes for such resistance could be very easily transmitted to the new virus.
According to the most recent figures posted this morning, the U.S. has so far seen 43,771 laboratory-confirmed cases of H1N1, and 302 deaths, "but that is just the tip of the iceberg," Schuchat said. More than a month ago, the agency estimated that more than 1 million Americans had been infected, but that estimate has not been updated.
Friday morning's posting of confirmed cases will be the last, she added, because the CDC wants to de-emphasize testing for the virus.
Influenza viruses mutate so freely that it is extremely difficult to make predictions.
Schuchat noted that this spring, 6% to 8% of the population in many U.S. communities were infected by the virus "in a season when transmission conditions weren't that great."
With the longer winter season coming, "we think it will reach two to three times that number." Normally, 10% to 15% of people in a community are infected with seasonal flu.
And with closures due to outbreaks, as well as the need for many healthy adults to stay home and take care of a spouse or child with the flu, as much as 40% of the workforce might be unable to work at the pandemic's peak, she noted.
Deaths caused by H1N1 flu over the two-year period could range from 90,000 to several hundred thousand, depending on the efficacy of the swine flu vaccine, officials project.