Vaccines

Vaccines help reduce the risk of a disease by introducing a dead or weakened version of the disease-causing organism to the body’s immune system. If our immune system then encounters the ‘real’ disease-causing germ, quick recognition allows our body to fight infection with a rapid and effective immune response. In this way, vaccines mimic the body’s natural immunity.

Immunisation is the most effective preventative measure against infectious diseases. Some vaccines offer lifelong immunity. In other cases, booster shots are needed.

Vaccines and immunisation carry a small degree of risk, but this must be weighed against the risk of the disease. Some diseases, such as poliomyelitis or diphtheria, can be so destructive that the infected person may suffer lifelong complications or even die.

How vaccines work
The human immune system is a collection of special cells and chemicals that fight infection. The human body can gain immunity against certain diseases either naturally (by catching and surviving, or coming into contact with, the illness) or through immunisation.

Vaccines work by introducing a modified version of a disease-causing microbe (germ) to the immune system, either by injection or within a small drink. Depending on the disease, the vaccine could be a weakened or dead microbe or a section of a microbe (for example, its protein wall). In other cases, where disease is caused by toxins produced by bacteria, the vaccine contains the deactivated toxin.

The immune system responds to the weakened, partial or dead microbe as if it was a fully-fledged germ and makes antibodies to destroy it. These antibodies are made without the person getting sick.

Later, some of these antibodies remain ‘on patrol’ in the bloodstream and can increase quickly if the germ is encountered. This means that the key to defeating that particular disease is stored long term. This is called ‘immunological memory’.

Vaccines mimic natural immunity
A person’s natural immunity is like a library and contains information on every microbe ever defeated by their immune system. If the person encounters the real disease-causing microbe in the future, their immune system will know how to defeat it, often before the person experiences any symptoms of illness. This explains why (in most cases) diseases such as mumps and rubella (German measles) are only caught once, even though exposure to the virus may be repeated throughout life.

Vaccines are designed according to how particular germs make you sick. For example, the reaction to measles is to the whole organism. However, in tetanus, the body reacts to the poison produced by the tetanus germ, rather than the germ itself.

History of immunisation
Modern immunisation techniques were pioneered over 200 years ago, when smallpox was a feared and deadly disease.

An eighteenth century doctor named Edward Jenner noted that farm labourers who contracted the mild disease cowpox were immune to smallpox. Jenner guessed that the germ responsible for cowpox was similar enough to the smallpox microbe to ‘train’ the immune system to defeat both diseases. He was right. Immunisation today relies on broadly similar techniques.

No vaccine is completely effective
Out of 100 people immunised against a particular disease, about five to 15 will catch the disease if exposed to it. This may be because their immune systems didn’t respond to the vaccine and failed to make antibodies or the vaccine may not have been stored correctly at the clinic.

However, if (for example) 100 people who have never had measles or been immunised against it were exposed to the disease, almost every single one of them would catch it. Immunisation greatly reduces the risk of catching a disease, which in turn reduces the risk of complications.

Comparing the risks
The safety of vaccines has been questioned from time to time. Like any other medicine, vaccines carry a small degree of risk. This must be weighed against the risk of the disease. For example, the risk of encephalitis (inflammation of the brain) from the measles-mumps-rubella (MMR) vaccine is thought to be just one in a million immunisations. However, according to some estimates, the risk of encephalitis from catching the mumps virus is one in 200.

Immunisation success stories
Immunisation is the most effective preventative measure against infectious diseases. Some recent immunisation success stories in Victoria include:

• Pneumococcal disease – following the introduction of the pneumococcal vaccine for all infants in 2005, the number of cases has decreased from an average of 94 cases per year in the three years before the immunisation program began to five cases in 2007 (in children up to four years old).
• Meningococcal C disease – from a peak of 88 cases in 2002, the number of cases has fallen every year since the introduction of meningococcal C vaccine in 2003. In 2007 there were only two cases notified in Victoria.
• Haemophilus influenzae type b (Hib) disease – since immunisation against Hib disease was introduced into Victoria in 1993, cases among children under four years of age have dropped by more than 99 per cent. In 1991–2 there was an average of 110 cases per year. This fell to less than one case per year for the period from 2000 to 2007.
• Measles – measles rates have been decreasing since the vaccine has been available in Australia (since 1969). The most significant impacts have been from its inclusion on the National Immunisation Program in 1983, the introduction of a second measles vaccine dose in 1994 and a primary school Measles Control Campaign in 1998. In Victoria, the average number of measles cases has decreased from 75 cases per year between 1997 and 1999 to five cases per year from 2005 to 2007. The most common age of people seeking treatment for measles since 2000 (more than two-thirds of cases) are those who were born after 1966 (who were not exposed to the pre-vaccine era of large outbreaks) and people born before 1982 (who were not vaccinated or only received one dose).

• Your doctor
• Your local council immunisation service
• Maternal and Child Health nurse
• Nurse-on-Call Tel. 1300 606 024 – for expert health information and advice (24 hours, 7 days)
• Immunisation Program - Department of Health Victoria, Tel. 1300 882 008
• National Immunisation Infoline Tel. 1800 671 811

• Vaccines help reduce the risk of certain infectious diseases by introducing dead or weakened versions of disease-causing organisms to the immune system.
• Some vaccines offer lifelong immunity.
• Immunisation greatly reduces the risk of catching a disease, which in turn reduces the risk of complications.

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