Making a vaccine against seasonal influenza is a constant catch-up game. Scientists must predict which of the constantly mutating virus strains will be most virulent six months in the future, the amount of time it takes to manufacture the vaccine. The system has worked well enough for the regular flu. But when new, virulent strains emerge--including the current, rapidly spreading swine flu (H1N1)--the traditional approach falls short. Even as consumers clamored for a vaccine, it took seven months and around 48,000 confirmed U.S. cases before the first H1N1 vaccines were shipped to hospitals around the country.

Influenza vaccine production has not changed substantially since it was first introduced in the 1940s. The new H1N1 vaccine took so long to make because it was manufactured using the usual technique--vaccine specialists identify and isolate the most virulent strains, weaken them, genetically adapt them for growth in birds as well as in mammalian cells, and then inject them into fertilized chicken eggs, where the virus can reproduce without killing its host. Once inactivated, the viral proteins can then be made into a vaccine. Add quality control and distribution, "and it is a five-to-six-month process, at its best," says Gregory Poland, director of the Mayo Clinic's Vaccine Research Group, in Rochester, MN.

Researchers are working hard to develop faster production methods for seasonal flu vaccines as well as for "universal" vaccines that could guard against almost all influenza strains, including swine and avian. But both are a long way down the road. "There is really nothing in the pipeline that will guarantee the production of vaccine in much less than six months," says Robert Webster, an infectious disease and viral immunology expert at St. Jude Children's Research Hospital in Memphis, TN.

Some companies, including Novartis and Baxter International, are working on flu and other vaccines that could be grown in cell culture rather than in eggs--a method that has the potential to halve time to production. The time-consuming steps of tweaking the virus strains so they'll grow in bird rather than mammalian cells, and weakening them so that they can reproduce without killing the egg, would no longer be required. And manufacturers would no longer be dependent on the available egg supply. "With cells, you can grow them up, freeze them, and bring them out when you need them," Poland says. "You can make as much or as little as you want."

Both Novartis and Baxter have clinical trials under way, and Baxter just received European marketing approval for its H1N1 vaccine. But the process could take much longer in the U.S. because the cell-culture method itself has not been approved by the Food and Drug Administration. Companies will have to go through testing and manufacturing inspections that will cost on the order of about $500 million each, says Poland.

Other researchers are looking beyond single strains of influenza and into the possibility of creating a vaccine that can protect against almost all versions of the virus. Polio and measles vaccines given in childhood confer a lifetime of immunity because the viruses they protect against change very little from year to year, but the flu virus mutates fast, changing its outer proteins almost completely every season. However, researchers have found a few stable regions on the virus that they believe could be used to create a vaccine that could guard its recipients against nearly all strains of influenza, including those most likely to cause a pandemic.