Vaccines are a central player in our fight against infectious diseases. What components are commonly found in vaccines, and what is their purpose? In this Special Feature article, we find out.

Why do some vaccines have a long list of components?

Many people will be familiar with the concept that a vaccine against a particular virus will contain a small amount of the pathogen or a part of it, at least.

When we receive the vaccine, the viral interloper triggers our immune system to launch a series of events that leave us protected against the pathogen in the future.

 But a glance at the ingredients in common vaccines reveals a long list of other components, the roles of which might not seem so clear cut.

 What is the purpose of the likes of gelatin, thimerosal, and Polysorbate 80? And why do some vaccines contain aluminum?

In this Special Feature article, we look at the active and inactive ingredients that make their way into vaccines and reveal what their role is in protecting us from infectious diseases.Our immune system and active ingredients

The active ingredient in a vaccine is usually made from the viral or bacterial pathogen itself. There are two different approaches to this, with the pathogen being either alive or inactivated.

Vaccines that incorporate living bacteria or viruses are called live attenuated vaccines. The pathogen is weakened to prevent it from causing the disease, but it is still able to elicit a strong immune response.

 Live attenuated vaccines work very well, but they are not suitable for everyone. If a person is immunocompromised, they may contract the very disease from which the vaccine should be protecting them.

Many vaccines, therefore, use an inactivated version of the active ingredients, which can take the form of whole bacteria or viruses that have been killed.

 However, most vaccines are actually acellular, which means that they do not contain the whole pathogenic organism. Instead, they are made from parts of the pathogen, such as proteins or sugar molecules. Our bodies recognize these molecules as foreign and mount an immune response.

 Examples of acellular vaccines are:

toxoid vaccines that contain inactivated toxins from pathogenic bacteria

conjugate vaccines made from a combination of pathogen-specific sugar molecules and toxoid proteins, as the sugars themselves do not cause sufficiently strong immune responses

recombinant vaccines made by using bacteria or yeast cells to make many copies of specific molecules from the pathogen

Aside from the active ingredient, vaccines contain many other things. The technical term for these is excipients.