Washington ? 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, federal 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 doesn’t 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.