Scientists decide the best approach to design a vaccine depending on the disease-causing agent, how it infects the cell and how the immune system responds to it. The following are the main type of options that currently exist:
These contain whole bacteria or viruses that have been “weakened” so that they create a protective immune response but do not cause disease in healthy people. Live vaccines tend to create a strong and lasting immune response, however they are not suitable for people whose immune system does not work either due to drug treatment or underlying illness. Examples of live vaccines include: The varicella-zoster vaccine, oral poliovirus (OPV) vaccine and yellow fever virus vaccine.
To produce this type of vaccine, bacteria or viruses are killed or inactivated by a chemical treatment or heat. Inactivated vaccines are suitable for healthy individuals, as well as people with weakened immune systems as they do not contain live forms of the microorganism which they are designed to protect against. Inactivated vaccines do not always create a strong or long-lasting immune response, in the same way as live vaccines, so they usually require repeated doses or booster doses. Examples of inactivated vaccines include: inactivated poliovirus (IPV) vaccine, whole cell pertussis (whooping cough) vaccine, rabies vaccine and the hepatitis A virus vaccine.
These vaccines do not contain any whole bacteria or viruses at all. Instead they contain some components, such as polysaccharides (sugars) or proteins, from bacteria or viruses. These are the parts that our immune system recognizes as foreign, they are the antigens that trigger an immune response. Even though the vaccine might only contain a few of the thousands of proteins in a bacterium they are enough in themselves to trigger an immune response which can protect against the disease. This method of creating vaccines is used for the Haemophilus influenzae type b vaccine and the acellular pertussis (whooping cough) vaccine.
Some bacteria release toxins (poisonous proteins) when they attack the body. The immune system recognises these toxins in the same way it recognises the antigens on the surface of bacteria. Some vaccines are made with inactivated versions of these toxins. They are called “toxoids” because they look like toxins but are not poisonous. Examples of vaccines utilising this approach include the diptheria and tetanus vaccine.