ATLANTA - With two flus likely in fall, most people will need three shots: one for seasonal influenza and probably two others to protect against the new swine variety.

Experts suggest you get that first shot as early as this month - if you can find it.

"We'd like to get to Job 1 and get most of it done," said William Schaffner, a Vanderbilt University flu expert. "Get it done before we start to tackle Job 2" - the more complex task of swine flu vaccinations.

The five vaccine manufacturers that supply the United States are finishing up production of seasonal flu vaccine earlier than usual. Health officials say they expect about half of the more than 120 million doses of seasonal vaccine to be available by the end of this month. Most of the rest are due out by the end of September. Some manufacturers report that distributors are quickly buying up supplies.

Those five companies are the same ones making the new swine flu vaccine. They are on track to start delivering the first batches in September, but most isn't expected until late October or November, health officials say.

Why can't you get one shot for all - or maybe just two?

The reasons have to do with logistics and caution.

Scientists believe the swine flu vaccine will be most effective if given in two doses, about three weeks apart, although testing is still under way to check that.

Combining swine flu and seasonal flu in one shot is theoretically possible, but it was too late to try it this year. So they will have to be given as separate doses, even if it's at the same appointment.

And the government has different lists for who should be first to get each: for the regular flu shot, elderly people, health-care workers, and pregnant women are among the priority groups; for H1N1, health-care workers and pregnant women are on the list but not older people, who may have some immunity.

If all the flu shots were given about the same time, it could mean a mash of people, some of whom should be among the first to get one shot and not the other. Then there are safety questions.

Health officials are haunted by the swine flu vaccine campaign in 1976, which was stopped after unexpectedly high numbers of people suffered a paralyzing condition called Guillain-Barre Syndrome. While it's not clear the vaccine was to blame, officials want to carefully monitor people who get the new vaccine for any problems.

Scientists are just beginning to test its safety and effectiveness, work that is likely to take months. If the seasonal and swine flu vaccines were given at the same sitting and some people developed health problems, it would be hard to single out which caused the problem, or whether it was the combination.

"How you're going to separate that out - that's a doozer," said Samuel Katz, a Duke University vaccines expert.

So the government is looking at three shots, preferably over three visits.