The computer model developed by Germann's team combines information about influenza transmission with U.S. population and transportation data to study how a virus could spread.
The model assumes a human-transmissible virus will be introduced to the U.S. via international air travel.
The model's findings include these projections:
• Due to the high frequency of domestic travel, the virus could be seeded in communities nearly simultaneously across the country once it arrives in the U.S.
• Given how humans interact in their homes, schools, places of work, shopping malls, and on the occasional long-distance trip, a highly transmissible strain would create a pandemic within 24 days.
• A containment strategy that included travel restrictions, widespread vaccinations, and the development of a vaccine targeted to the specific strain could limit the number of people infected to less than 10 percent of the U.S. population.
• While restrictions such as school and airport closures would not by themselves limit the number of people infected, such measures could buy researchers time to develop a specific vaccine based on the pandemic virus.
Exactly what measures should be taken will depend on the transmissibility of the virus, Germann said. This factor cannot be determined until an outbreak occurs, and even then it is difficult to gauge.
"How [transmissibility] is determined is still somewhat controversial," he said.
In addition, he added, before officials can put a vaccine plan in place, the U.S. needs to stockpile a so-called best-guess vaccine for the most common flu strains, and must develop the capacity to produce targeted vaccines rapidly once an outbreak occurs.
Neither precaution currently exists in the U.S. for a highly transmissible strain, Germann said.
Without any vaccines or interventions such as travel restrictions in place, he said, the U.S. would need 182 million courses of the antiviral drug oseltamivir (sold under the trade name Tamiflu) to combat a virus with the same transmissibility of the virus that caused the 1918 pandemic.
The current national stockpile of Tamiflu stands at about 5 million courses.
The results of the computer-model study appear in this week's issue of the Proceedings of the National Academy of Sciences.
Sandrock, who was not involved in the study, said the model does a "good job" at representing the virus spread and potential containment strategies.
But, he cautioned, "nothing is uniform in life as compared to a mathematical model."
Vaccine Development
A second team reports that there is some promise for the development of a bird flu vaccine.
According to a study in last week's New England Journal of Medicine, scientists have developed an experimental bird flu vaccine for humans that protected about half the people vaccinated.
But the vaccine requires a dose up to 12 times larger than a regular flu shot. At that dosage, the vaccine is too difficult to mass-produce, the team said.
"We have a long way to go," Anthony Fauci, director of the National Institute of Allergy and Infectious Disease (NAID), told reporters in a telephone conference last week, according to news reports.
NAID, a division of the U.S. National Institutes of Health, funded the research.
Lead investigator John Treanor of the University of Rochester in New York and his colleagues are now attempting to stretch the vaccine with an adjuvant, a substance that enhances the response of the immune system.
NAID is also looking at ways to speed the vaccine manufacturing process, as well as new ways to deliver vaccines.
According to Sandrock, the pandemic response system as a whole is not yet ready. But, he said, "We are getting there."
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