4 Strategies for vaccine production

4.1 Introduction

The type of antigen preparation used in active vaccines varies considerably, depending on the pathogen. For diseases such as tetanus, where a bacterial toxin causes the damage, the toxin is first chemically treated to turn it into a harmless toxoid, which is used as the immunising agent. Where protection is required against the pathogen itself, vaccines are based on whole organisms treated in some way to make them safe, or on complex mixtures of antigens taken from the infectious agent. In this section, we discuss the major production strategies for active vaccines, which conventionally contain one or a combination of:

• intact killed pathogens;

• live attenuated pathogens;

• subcellular fragments or molecules from the pathogen – known as subunit vaccines – either alone or linked in ‘conjugates’ to other molecules.

Table 3 gives some examples of commonly used vaccines in each of these categories. The difficulties that can be encountered in developing an effective vaccine are well illustrated in Section 4.2 of the Cholera Case Study (see Section 4.2, which describes some of the many attempts to produce vaccines of all three types. Sometimes vaccine preparations consist of a mixture of components, as in one of the newer vaccines against cholera, which contains inactivated classical and El Tor bacterial strains and a component of the cholera toxin.

In addition, we will look briefly at a number of new approaches to vaccine design currently in development, including:

• DNA vaccines containing ‘naked’ DNA encoding specific pathogen antigens;

• Genetic engineering of genes coding for key pathogen antigens either as subunits, or for cloning into non-pathogenic infectious agents used as ‘gene vectors’ for expression in the vaccine recipient.