LAURAN NEERGAARD AP Medical Writer WASHINGTON (AP) — The U.S. may have as many as 160 million doses of swine flu vaccine available sometime in October, even though manufacturers worldwide are having serious trouble brewing shots, government health officials said Thursday. The Food and Drug Administration may formally approve much of that vaccine before studies required to prove how well it works are completed, treating the new inoculations just like the recipe change that regular winter flu vaccine undergoes each year. That does not mean mass vaccinations would start before key information from studies of thousands of volunteers is in, U.S. officials stressed Thursday. Most of those studies start next month to determine if people will need one shot or two for good protection and how high a dose should be in each shot. The earliest results should start arriving in September and October. But the FDA told its scientific advisers it could finish the red tape of licensing much of that vaccine well before the use-it-or-not decision is made — because it’s brewed exactly the same as regular winter flu vaccine, merely using the new swine influenza virus, part of the common H1N1 influenza family, as the chief ingredient. Companies just have to take the normal steps required for each year’s regular winter flu vaccine, such as proving the inoculations are manufactured appropriately. Taking the same path now will save some important time because “the virus is ahead of us,” said FDA vaccine chief Dr. Norman Baylor. This “is not a rubber stamp. We do need to review some data to give us some comfort that that vaccine will provide some benefit and that it’s manufactured properly.” Even if swine flu is not more dangerous than regular winter flu, “we have a population that’s virtually 100 percent susceptible,” said committee chairman Dr. John Modlin, a pediatrician at Dartmouth-Hitchcock Medical Center. “This is an entirely appropriate way of proceeding, one that to me seems both necessary and appropriate and prudent.” European regulators plan to license swine flu vaccine very similarly, although the U.S. is awaiting study data before deciding whether to use it. Make no mistake: Vaccines containing immune-system boosters called adjuvants are not candidates for the easier strain-change approval, the FDA said. Flu vaccine with this extra ingredient is widely sold in Europe but never has been sold here, and there’s little information about their safety in young children or pregnant women. While both adjuvant-free and adjuvant-added swine flu vaccine is being tested in the U.S. and abroad, using it outside of those studies would require a completely separate government decision. Even if 160 million doses are in hand in October, at the expected two doses a person, that supply would cover only a fraction of the populations that this new influenza strain is sickening the most — school-age children, teenagers and young adults. More vaccine would trickle out over the following months. Companies making swine flu shots are struggling. The chief ingredient for influenza vaccine is grown in chicken eggs, and companies are getting far fewer doses per egg — 30 percent of the normal yield for regular winter flu vaccine, said FDA’s Dr. Jerry Weir. One bright spot: The U.S. has the world’s only nasal-spray flu vaccine, and maker announced Thursday that it’s producing plenty — so many millions of doses a month that it cannot keep up with putting them into the special sprayer required to use it. So MedImmune Inc. is working with the government to see if it can race out a different method for fall, simply dripping its swine flu vaccine into people’s noses. “A dropper instead of a sprayer works as well,” said MedImmune vice president Dr. Ben Machielse. The U.S. estimates for its October supply took that problem into account, said Robin Robinson of the Department of Health and Human Services, which is buying the nation’s swine flu supply and will decide who receives it. But that estimate also assumes that two low-dose shots, about a month apart, will offer enough protection. If studies show people need higher-dose shots, that will further cut the early available supply. Not included in that calculation are MedImmune’s potential extras. It used a different “seed virus” to grow vaccine than other manufacturers, because it’s a different type of vaccine: Flu shots are made of killed influenza virus, while FluMist is a live but weakened strain. It comes in a set dose, and MedImmune said it will have 14 million swine flu nasal sprays available by October, and 40 million by year’s end. But overall it’s producing roughly 35 million doses a month, Malchielse said, if only it could turn that bulk product into a form easily administered to people. The good news: Despite a brisk flu season in the Southern Hemisphere, the new swine flu is not yet mutating to become more dangerous, said Dr. Nancy Cox of the Centers for Disease Control and Prevention. Cox said “it’s actually quite surprising” that the virus is showing so little genetic variation given its rapid spread. It has sickened more than a million people in the U.S. alone since April and circled the globe in a matter of weeks.