Other than the antigen, a couple of things have to be included in the vaccine in order for it to be effective. The requirements vary depending on the specific vaccine, but the gist is the same.

The vaccine has to be stable because it leaves the manufacturing plant, gets bounced around on trucks and so forth. Sometimes small chemicals are added to act as stabilizers so that the vaccine material remains potent. These chemicals are thoroughly regulated by the Food and Drug Administration (FDA) to ensure their safety and are usually present only in trace amounts.

In multidose vials, a disinfectant is required. This is so that each time a dose is removed, any foreign matter that intrudes is killed instantly. Traditionally, Thimerosol has been the most popular among scientists. However, the industry has largely abandoned its use because of concerns that the chemical causes adverse reactions (see sidebar about vaccine myths). In fact, multidose vials are being phased out in favor of single-dose vials, even though these are more expensive. Thimerosol is currently present only in trace amounts in the influenza vaccine, but that will be a thing of the past in a few years. "We would like to put the controversy behind us," says Dr. Schaffner. "None of us associated with vaccines believe there is merit to that belief, but we are going the extra hundred miles to reassure parents."

On the next page we'll find out how vaccines are made.