ATLANTA (AP) -- A potential fall swine flu immunization campaign may involve an unprecedented 600 million doses of vaccine, though officials said Friday they haven't figured out how to administer so many doses or accurately track side effects if a seasonal vaccine is given simultaneously.

The swine flu campaign could far eclipse the roughly 115 million doses of seasonal flu vaccine distributed each year, officials said at a national vaccine advisory committee meeting.

No final decision has been made about whether a swine flu vaccination campaign will take place or whether all Americans would get immunizations. Health officials said that a swine flu vaccination campaign could be only a few months away, and that as many as 60 million doses could be ready by September. The timing depends on how fast a vaccine can be produced and tested, however.

However, health officials are clearly getting ready for a massive vaccination effort, and worry that illnesses could continue or even accelerate in the fall or winter. Preparation discussions dominated a three-day meeting in Atlanta of the Advisory Committee on Immunization Practices, a panel that guides U.S. vaccination policy.

The virus already has caused at least 27,000 illnesses and 127 deaths in this country. Twelve states are seeing widespread cases, and about 6,000 cases were reported in the past week -- more than in any other week since swine flu first appeared in late April.

Those are just reported cases. More than a million U.S. infections probably have occurred, said Dr. Anne Schuchat of the U.S. Centers for Disease Control and Prevention.

"This new infectious disease is not going away," said Schuchat, director of the CDC's National Center for Immunization and Respiratory Diseases.

A new, small CDC study found the virus can cause more serious disease than seasonal flu in ferrets, which have a similar respiratory system to humans. But it does not seem to spread as easily, at least through the droplets that infected ferrets sneeze or cough into the air, said Dr. Nancy Cox, a CDC flu scientist.

However, officials are still worried about the seasonal flu, which causes an estimated 200,000 hospitalizations and 36,000 deaths each year.

Five flu vaccine manufacturers are producing 120 million doses for the 2009-2010 flu season, with a third of that available by Sept. 1 and most of the rest shipped by Nov. 1, CDC officials said. Federal officials are working to ensure a swine flu campaign doesn't force the manufacturers to scale back production of the seasonal vaccine.

The swine flu campaign could be a huge undertaking, involving as many as 600 million doses. Although about 300 million people live in the U.S., health officials anticipate children and perhaps adults under age 50 may need two doses each.

However, the logistics of such a campaign are still being worked out. The CDC relies on one company to distribute 80 million publicly financed vaccine doses for children. That company, McKesson Specialty, still has not said whether it could handle distributing as many as 600 million doses of swine flu vaccines to clinics and doctors around the country, said Dr. Jeanne Santoli, who oversees vaccine purchase and distribution for the CDC.

Officials said they'll probably need to recruit physicians who don't usually give flu shots because local health departments have cut more than 10,000 jobs. Tracking side effects could also be tricky.

Problems don't always show up in early studies. And if the vaccine is given at the same time as the seasonal flu shots, it could be difficult to figure out which vaccine is causing problems.

That happened in 1976, when officials vaccinated 40 million Americans in anticipation of a new strain of swine flu. But the pandemic never materialized, and at least 500 people who got the shots came down with a paralyzing condition called Guillain-Barre Syndrome. It's still unknown what caused the condition.

Some health officials said they'd rather start a swine flu vaccine campaign a few months after the seasonal shots to simplify side-effect monitoring. But they may not have a choice, said Cox, the CDC flu scientist.

Swine flu and seasonal flu have been circulating together, with new data from Australia -- which is experiencing its annual flu season -- indicating that flu cases there are 60 percent swine and 40 percent seasonal.

"There are too many complexities" to say what will be done in the fall, Cox said. "We'll have to wait and see."

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