The Obama administration is considering an unprecedented fall vaccination campaign that could entail giving Americans three flu shots -- one to combat annual seasonal influenza and two targeted at the new swine flu virus spreading across the globe [ or so they say ].

If enacted, the multibillion-dollar effort would represent the first time that top federal health officials have asked Americans to get more than one flu vaccine in a year, raising serious challenges concerning production, distribution and the ability to track potentially severe side effects.

Another option, said Dale Morse, chairman of the advisory committee on immunization practices at the Centers for Disease Control and Prevention, is adding to the seasonal flu shot an ingredient targeted at the new virus.

Experts in and out of the administration are evaluating a raft of complicated issues, including who ought to receive an inoculation against the swine flu and whether private vaccine makers can simultaneously manufacture the standard 180 million doses as well as up to 600 million rounds of a new vaccine.

"We are moving forward with making a vaccine," said Robin Robinson, a director with the Department of Health and Human Services who oversees pandemic response programs. Robinson said that although a formal decision about the swine flu vaccine has not been made, if the government goes ahead, it would probably produce two doses for all Americans. If the threat diminishes, he said, health officials could decide to produce doses for only a portion of the population.

Vaccine and pandemic experts are working with the administration to determine how to produce, test, track and educate the public about two different influenza vaccines in the same flu season.

"They have never tried this before, and there is going to be a great deal of confusion," said William Schaffner, chairman of the Department of Preventive Medicine at Vanderbilt University School of Medicine.

Memories of the nation's earlier experience with a swine flu vaccine present another challenge. In 1976, hundreds of Americans developed neurological disorders after they were vaccinated for a swine flu strain. The public was asked to receive one of two vaccines developed to combat the strain.

Health officials have asked manufacturers to ramp up production of the seasonal vaccine scheduled for rollout this fall to make way for the possible mass production of a swine flu vaccine.

A decision on whether to produce such a vaccine will have to be made soon, because it typically takes five months to produce a new vaccine and authorities would want it available for the next flu season.

Some medical experts said rolling out two vaccines would present additional challenges in terms of testing and tracking adverse reactions. Health officials and manufacturers will need to know what the negative reactions might be for each vaccine on its own and in combination with the other. Initial tests would be done on animals, and then clinical trials would be conducted with people to determine side effects before either vaccine is rolled out.

Harvey Fineberg, president of the Institute of Medicine, part of the National Academy of Sciences, said officials will have to weigh the risks of the time spent on testing.

"All this takes time, money and organization," said Fineberg, who led an investigation into the government's handling of the 1976 swine flu vaccinations.

The greater challenge will be tracking any adverse reactions as millions of Americans get multiple vaccinations in a matter of months this fall and winter.

"There will be adverse effects to any vaccine. That's just science," said Michael Hattwick, who ran the CDC's vaccine-tracking system during the last swine flu scare.

Hattwick said a "real-time" tracking system would need to be established to provide constant updates to the CDC about adverse reactions. That information, he said, should include lot numbers for the vaccines so health officials can trace each side effect to the manufacturer and the date of production. Routine flu vaccinations are not traced with such precision because reporting is voluntary and often delayed, Hattwick said.

Anthony S. Fauci, director of the National Institute of Allergy and Infectious Diseases, said he does not expect additional adverse reactions with two vaccinations. The traditional flu vaccine is designed to attack the three flu strains health officials believe to be the greatest threats in a regular season, he said.

"In a regular seasonal flu, you get three vaccines. Adding an additional one should not present a problem," Fauci said.

A record-keeping system would also need to be devised to track which doses patients have received, health experts said.

Without such a system, patients could lose track of which of the three shots they have received or could fail to get the second swine flu inoculation at the proper time.

"We will have to keep them straight and separate," Vanderbilt's Schaffner said. "This will be an enormous challenge, and we haven't figured out how to do it yet. That's one of the things we are trying to sort out."