Why No More Flu Vaccine?

For one thing, there’s a shortage of chicken eggs.

The current shortage of flu vaccines has health officials worried about the possibility of a major epidemic. But manufacturers say it will take six months or more to produce additional vaccines. Why does the process take so long?

Because specially purified and fertilized chicken eggs—the kind manufacturers grow the vaccines in—are hard to come by. There are two stages to creating the influenza vaccine each year. First, researchers working with the Food and Drug Administration and the World Health Organization predict which three strains of the flu will hit heaviest in the coming winter. Then, they develop three seed strains—benign versions of each virus for inclusion in the vaccine.

When the seed strains are ready, they need to go forth and multiply. But viruses can’t reproduce on their own: They require a host organism, and that’s where the eggs come in. The FDA sends the seed strains to manufacturers, who inject them into millions of specially purified fertilized chicken eggs. (Check out this video on the process.) Each egg incubates just one of the three seed strains, which replicate themselves vigorously therein. Then the strains are harvested, tested, and blended together to create the season’s influenza vaccine cocktail. All in all, the process can take six months or more.

The hitch is this: In order to get the millions of necessary fertilized eggs laid and delivered on schedule, at exact intervals from February to August of each year, manufacturers must employ considerable logistical savvy. Securing enough eggs can take over a year of advance planning. And once the egg order for a given year is locked in, there’s no way to ramp up production. (There’s no turning to the grocery store for help, either—the eggs there, though plentiful, are unfertilized.) So, even if a new batch of vaccines were started tomorrow, they probably wouldn’t be ready until our flu season is over. In any case, manufacturers are already busy preparing for the flu season in the southern hemisphere.

Flu vaccines have been manufactured this way for decades. But the crisis has amplified calls for overhauling the system. The process of developing seed strains is currently very time-consuming, and some scientists argue they should be developed using “reverse genetics,” which eliminates some trial and error. The manufacturing process could also be revamped, possibly by using tissue cultures instead of eggs. Tissue cultures are an industrial-scale version of the petri dish: The seed strains multiply when they’re injected into a mammalian cell. (The favored cell for this sort of work, called “Vero,” is derived from an African green monkey’s kidney.) Tissue cultures are already used to make vaccines for some other diseases, including rabies.

Next question?

Explainer thanks Dr. Richard Webby of St. Jude’s Children’s Research Hospital, the Centers for Disease Control Flu Web site, and Aventis-Pasteur’s vaccine manufacturing site.