Toxoid and subunit vaccines use parts of the microbes instead of the whole organism.
Characteristics of toxoid vaccines include:
- Toxoid vaccines are used for diseases that are caused by a toxin produced by a bacterial pathogen and are formulated by purifying the toxin and deactivating it with heat or chemicals.
- The immune response is therefore to the toxin, not to the bacteria.
- Toxoid vaccines are extremely safe and effective; they should be stored in a refrigerator.
Subunit vaccines are formed from parts of the microorganism that create very specific antigens to induce an immune response. Some of these, such as conjugate vaccines and virus-like particles, are listed separately on the chart on page 15, but they are all considered subunit vaccines. Examples of the various antigens used in subunit vaccines are:
- Polysaccharides (e.g., components of bacterial capsules)
- Polysaccharides conjugated to protein components (to increase the immunogenicity)—known sometimes as conjugate vaccines
- Recombinant protein vaccines; for instance, DNA of the Hepatitis B virus inserted into a yeast cell which then synthesizes the antigen of interest
- Virus-like particles such as those components of the outer layer of a virus
- Outer membrane vesicles of bacteria
One thing to be noted about many of the subunit vaccines is that the antigen (the subunit) may not be strong enough by itself to elicit a good immune response. Therefore, these vaccines must incorporate an
adjuvant which is a substance that enhances the immune response. Examples of adjuvants are aluminum, liposomal formulations, cytosine phosphoguanine (CpG), saponins, and oil-in-water emulsions. Many of these are known simply by numbers and letters such as AS01g, MF59, etc. Most subunit vaccines must be refrigerated when stored.
