Now the question is whether vaccine manufacturers can make such a big change in time to produce more than 100 million doses by the fall.
"It's going to be a really busy spring and summer, and of course we're always looking for fallback positions just in case things don't work out well," said Dr. Nancy Cox, flu director at the Centers for Disease Control and Prevention. "There's a lot of work that will be going on ... to try and make sure that everything comes together in such a way that there will be plenty of vaccine."
One concern: A strain called Brisbane/10 that's responsible for much of this winter's misery doesn't grow very quickly in the laboratory, potentially complicating already laborious vaccine production.
The flu vaccine must be reformulated every year to keep up with the fast-evolving influenza virus, and this year the government made a rare wrong bet on which strains would cause the most disease. The flu season got off to a slow start, but it rocketed in mid-January as some new strains arrived -- and the CDC found the vaccine is a good match for only about 40 percent of the virus now spreading in the U.S.
That Brisbane/10 strain is the big culprit, one first spotted in Australia late last winter, too late for scientists to include in this year's vaccine recipe even if they had predicted it would gain steam.
Flu viruses come in different strains that constantly mutate, until one that few people have immunity against emerges and is able to spread widely. Each year's vaccine contains protection against two varieties of the harsher Type A flu -- subtypes known as H1N1 and H3N2 -- and one from the more benign Type B family.
CDC and international authorities expect Brisbane/10, a version of the H3N2 flu, to still be around next year. They predict a second new Type A strain, known as H1N1/Brisbane/59, also will hit, along with a newer Type B/Florida strain, prompting Thursday's decision to put all three in next year's vaccine.
It's a gamble based on tracking illness around the globe, and the CDC does have a pretty good record: 16 of the last 19 flu seasons had well-matched vaccines.
Still, "as we always say, influenza is quite unpredictable," Cox cautioned Thursday.
The recipe must be set about eight months before manufacturers start shipping doses because flu vaccine production is so complex. Health authorities come up with seed stocks of the virus strains that manufacturers then must grow in chicken eggs.
Makers of vaccine for the Southern Hemisphere already have added the troublesome Brisbane/10 strain to their own shots, and found it doesn't grow easily. Scientists now are working on a solution to that technical problem, such as using a very similar virus -- it's even called "Brisbane/10-like" -- found in Uruguay that would provide the same protection, Cox said.
"It's certainly a challenge for all the manufacturers," said Nancy Kavanaugh of Medimmune Inc., which makes a nasal spray flu vaccine, the only non-shot version. "We're working with three new strains. ... There's some unknowns related to those and how they're going to grow."
As for the rest of this winter, the CDC says the current vaccine should provide some protection, perhaps resulting in a milder case of flu than if someone hadn't been inoculated. It's too early to tell if this winter's flu will be more deadly than usual. Every year, the flu infects up to 20 percent of the population, hospitalizes 200,000 people and kills 36,000.
