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Understanding Immunisation

There are several types of vaccines that are designed to produce an immune response. Vaccines are made differently depending on the characteristics of the infectious agent, whether it is a bacteria, virus or toxin.

Live attenuated vaccines are made up of living virus or bacteria that have been changed to weaken (attenuate) and reduce its virulence (ability to cause disease). The ‘wild’ viruses or bacteria are attenuated in a laboratory, usually by repeated culturing. For example, the measles vaccine used today was isolated from a child with measles in 1954. Almost 10 years of culturing was required to change the wild virus into vaccine virus. A live attenuated vaccine creates a good (cellular and antibody) immune response and often provides life-long immunity with only one or two doses. Certain vaccines against viral diseases such as measles, mumps, and chickenpox are made this way. As these vaccines are live, they are not suited for people with lower than normal immunity, such as people suffering from leukaemia or HIV infection.

Inactivated vaccines contain a virus or bacteria which has been killed by chemicals, heat or radiation so they cannot replicate and cause disease. Inactivated vaccines stimulate a weaker immune response compared to live vaccines and always require several doses to provideimmunity. Immunity from inactivated vaccines can reduce over time and booster injections may be required to maintain immunity.

Subunit vaccines are made from only part of the infectious agent. Scientists use the ‘part’ that best stimulates the immune system. As these vaccines only use part of the infectous agent the chance of a bad reaction to this kind of vaccine is lower.

Conjugate vaccines are very important, especially for babies. Some bacteria that can cause disease have a special coating that hide them from the immune system, especially immature immune systems of babies. Conjugate vaccines link these coatings to an ‘antigen’ or ‘toxoid’ that immature immune systems can recognize, respond and produce immunity.

Haemophilus influenzae type B (Hib) vaccine, pneumococcal vaccine and meningococcal vaccine are examples of conjugate vaccines.

Toxiod vaccines provide protection when the cause of an illness is the release of a toxin (harmful chemical) by bacteria. Scientists develop the vaccine from just the deactivated toxin (called toxoid), rather than the whole bacteria. A toxin can be deactivated by treating them with formalin, a solution of formaldehyde and sterilized water. Toxiods are safe for use in vaccines and the immune system produces antibodies that lock onto and block the toxin. Examples of toxoid vaccines include diphtheria and tetanus.

Recombinant vector vaccines are vaccine antigens produced by genetic engineering technology. The Hepatitis B vaccine and the Human Papillomavirus (HPV) vaccine which protects against cervical cancer, are developed this way.