The Hib vaccine contains chemicals from the outer covering of the Hib bacterium. Because these chemicals by themselves do not trigger the immune system to generate a response against the bacterium, parts of other bacteria are added to the vaccine to make it work better. In addition, a small amount of aluminum is added to the vaccine to further boost the immune system and enhance the effect of the vaccine. The concern about the aluminum additive is addressed in Chapter 4.

There are a total of five types of Hib vaccines on the market now. Three of them contain just the single Hib vaccine, and two others are combination vaccines. All Hib vaccines are made from a chemical found on the outer coating of the Hib bacterium. This chemical is called polyribosylribitol phosphate, or PRP. This chemical is a type of sugar, but it's not the same as the table sugar you are familiar with. This type of sugar is only found in the Hib bacteria, and it probably does not taste sweet.

The PRP component of the vaccine is always chemically connected to a piece of protein because the PRP component alone does not trigger a good immune response, and the Hib vaccine would not work without this additional piece of protein. There are three types of proteins that the PRP component is connected with. The PRP-OMP Hib vaccine is connected to the outer membrane protein of another bacterium called Neisseria meningitides. The PRP-T Hib vaccine is connected to a protein made by the tetanus bacterium. The HbOC Hib vaccine is connected to a protein made by the diphtheria bacterium.

The only purpose of the protein components in the various Hib vaccines is to alert the immune system to the presence of the PRP component of the Hib vaccine. You can think of the PRP component of the vaccine as a mug shot of the Hib bacterium, and the protein component of the vaccine is a big spotlight shining directly on the mug shot so that your child's immune system will not miss the mug shot. These additional proteins are not harmful; they are incapable of causing infection.

Beside the PRP and the protein components, all the Hib vaccines also contain a trace of aluminum. The aluminum is necessary because it further draws the attention of the immune system to the part of the body where the vaccine is injected and makes sure that the body does not overlook the mug shot of the bacterium. You can compare the role of the aluminum as sounding an alarm. After the injection is given, both the protein component of the vaccine and the aluminum help draw the attention of the immune system to ensure that the body's defense system takes a good look at the Hib bacterium and remembers it. The next time the real Hib germ tries to invade the body, an attack against it can be quickly mounted and the Hib will have no chance of establishing a successful invasion.

In the past, one form of the Hib vaccine used to contain the preservative thimerosal. However, the Hib vaccine that used to have thimerosal has been thimerosal-free since August 1999. The Hib vaccines made by other manufacturers never contained the thimerosal preservative.