FRIDAY, June 26 (HealthDay News) -- U.S. health officials said Friday that they are considering a swine flu immunization campaign that could involve an unprecedented 600 million doses of vaccine.

Still to be worked out is finding enough health-care workers to administer all those shots, and determining ways to record side effects if the vaccine is given at the same time as the seasonal flu vaccine. That could make it difficult to figure out which vaccine was causing the side effects, the Associated Press reported.

Meeting at a national vaccine advisory committee session in Atlanta, officials from the U.S. Centers for Disease Control and Prevention said the swine flu campaign could dwarf the roughly 115 million doses of seasonal flu vaccine and the 150 million doses of childhood vaccines distributed each year, the news service said.

Officials have not yet decided whether to start a swine flu vaccination program, or whether all Americans would get the shots, beginning in the fall. The timing depends on how fast a vaccine can be produced and tested; as many as 60 million doses could be ready by September. But a widespread vaccination campaign is likely, the AP said.

On Thursday, federal health officials said an estimated 1 million Americans have been infected with the H1N1 swine flu, which continues to produce mild illness and a fairly quick recovery in patients.

The estimate was based on mathematical modeling, Lyn Finelli, a flu surveillance official with the CDC, said at the vaccine advisory meeting, the AP reported.

Nearly 28,000 cases -- about half the cases in the world -- have been reported to the CDC, including 127 deaths. By comparison, an estimated 15 million to 60 million Americans are infected with the seasonal flu each year, leading to roughly 36,000 deaths.

Meanwhile, the head of the World Health Organization said Thursday that the previously undiscovered virus, which first surfaced in mid-April in Mexico, has yet to show any signs of mutating.

Health officials are closely monitoring the H1N1 swine flu virus as it migrates from the Northern Hemisphere to the Southern Hemisphere, where the flu season is now under way. Scientists are concerned the virus could mutate as it circulates around the world, becoming more virulent and posing a greater health threat.

"The virus is not mutating for the moment, it is stable," Margaret Chan, director general of the World Health Organization, said in Moscow, according to Agence France Press, citing Russian news agency reports.

Still, Chan underscored the need to closely monitor the virus' spread around the globe, adding that it was highly "unpredictable."

The WHO last week formally declared a pandemic, triggered by the rapid spread of the H1N1 virus across North America, Australia, South America, Europe and regions beyond.

What makes the H1N1 strain different from the typical seasonal flu is that about half of the people killed worldwide were young and previously healthy. In contrast, regular forms of the seasonal flu typically prove most lethal to the very young and the elderly.

Even though H1N1 swine flu infections continue to be mild, for the most part, health-care workers need to do more to protect themselves from infection by the virus. A small sample of 26 health-care workers found that half became infected while at work, according to a report in the June 19 issue of the CDC's Morbidity and Mortality Weekly Report.

"This includes one case where the exposure was to another ill health-care person," Dr. Michael Bell, the CDC's associate director for infection control, said during a June 18 press conference.

Bell said infection-control procedures need to be taken seriously.

More information

For more on swine flu, visit the U.S. Centers for Disease Control and Prevention.

SOURCES: June 18, 2009, teleconference with Daniel Jernigan, M.D., medical epidemiologist, Influenza Division, U.S. Centers for Disease Control and Prevention, and Michael Bell, M.D., associate director for infection control, Division of Healthcare and Quality Promotion, National Center for Preparedness, Detection and Control of Infectious Diseases, CDC; Associated Press; Agence France Press

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