LOS ANGELES — Imagine it is just after 11 a.m. on an autumn day sometime in the future and a flight from Hong Kong is touching down at Los Angeles International Airport.
Minutes later, hundreds of passengers walk off the 747 and head out into the nation's second most populous city, or they change planes and fly to other parts of the country or to Latin America and Europe.
Now imagine that several of the people who boarded the flight 13 hours earlier had been exposed to a flu virus before leaving Hong Kong, and the strain was significantly different from other influenza viruses that have circulated in recent years.
Within hours, a potentially fatal virus could spread to every corner of the globe and health experts would be living their worst nightmare: an influenza pandemic that could kill tens of millions of people.
The chief weapon against the virus is quick information: How fast will the infection spread? What parts of the world are going to be most seriously affected? What kind of vaccine is required? What group of people is most at risk?
In an effort to give a fighting chance to those who would deal with such a catastrophe, scientists at the Los Alamos National Laboratory in New Mexico and at the Centers for Disease Control and Prevention in Atlanta are using supercomputers to model an influenza pandemic and try to predict its course.
When the next generation of computing power comes on line at Los Alamos in just over two years, the model they hope to construct would run faster than real time and give public health experts at the CDC the chance to get ahead of the disease's course.
The project is not just a theoretical exercise. Pandemics--epidemics that spread worldwide--are rare, but they do occur every few decades, according to William Thompson, a CDC biostatistician working on the modeling project.
The last one, in 1918, was called the Spanish Flu and killed at least 20 million people worldwide, including 500,000 in the United States.
That was before jet travel created the potential to rapidly spread a deadly virus.
"Instead of taking a year to get around the world as it did in 1918, today it might take 24 hours," said Andrew White, director of Delphi Project, Los Alamos' effort to use supercomputers to create models and simulations.
Because the number of variables is staggering, tracking such a pandemic would require computers capable of incredible speed, 30 trillion calculations per second, or 30,000 times faster than the average PC.
Ultimately, the model's calculations would be able to take into account every person on Earth.
"We can do models that large," said Rob Farber, a technical staffer working on software that will drive the influenza models.
Farber said the variables include patterns of travel around the globe and the probability that those carrying the virus would infect others.
"People act in patterns," said Farber, suggesting that existing models of traffic patterns in big cities could be integrated into the influenza model.
"Moms take their children to school where they interact with others, and that is (part of) how the virus spreads," Farber said.
An infected person, or agent, Farber said, "can infect another person and make that person sick. That agent can get sick, or not."
One question the model would be able to address is how the spread of disease would be affected by the closing of certain air routes.
Farber said the model also could identify people likely to be exposed to new strains of the flu first and thus serve as early warning signals. He suggested that doctors and customs agents might be among such people.
As the model becomes more sophisticated, it would include data from the CDC on the historic mutation of the influenza virus, something that could help predict how serious a threat the emerging new mutation would represent.
If there were only minor mutations, people who had been exposed to similar flu strains earlier would have some immunity to the slightly altered strain. If a radically different kind of virus was detected, then there would be great risk.
"If it is very new then nobody's immune system has seen it and that is a scenario for disaster," Farber said. The first computer model will contain mutation patterns for influenza epidemics in the 1975-75 and 1980-81 flu season.
"The ultimate goal is to see if there is any predictability of the mutation of the virus," said Nancy Cox, a CDC scientist who oversees the center's influenza monitoring.
She said the influenza virus is composed of 10 genes and that one of those genes is important to the development of vaccines to combat the flu.
That gene has been isolated and thus monitoring it over time could provide vital information for those seeking to prevent the disease.
"It is very important to know how (the specific gene) is moving over time."
Despite the high hopes for the new computer model, some of those involved were anxious not to suggest that a safety net was about to be unfurled.
"A model is a model," Cox said. "It is only as good as the assumptions you put in it."