In 1995 the Australian Bureau of Statistics released a report showing that only 53 per cent of children aged between 3 months and 6 years were fully immunised against a range of potentially fatal diseases. This rate was one of the lowest in the developed world. A major government initiative – the Immunise Australia Program – has dramatically increased immunisation levels. A 2006 study showed that 91 per cent of 1-year-old children are fully immunised. Many childhood diseases can spread very quickly and have serious consequences. The improvement in the levels of immunisation should help Australia avoid new epidemics of vaccine-preventable diseases. The immune system To understand immunisation, we need first to understand the way in which the human body naturally protects itself against disease. Diseases come in many forms: some of the most lethal are caused by microorganisms such as bacteria, viruses and micro-parasites. To combat infection by these microorganisms, the body's immune system can marshal two main lines of defence – innate or natural immunity, and acquired immunity. These two lines of defence have different characteristics:
Vaccines
In 1796, Edward Jenner achieved protection against smallpox by infecting James Phipps with another strain of pox virus (Box 2: Smallpox – the eradication of a disease). This became the first widely used vaccine. More than 200 years later, highly effective and safe vaccines are available to protect against diseases caused by many viruses and bacteria (Box 3: The basics of making a vaccine). Some vaccines, such as many attenuated viruses (eg, yellow fever, measles) need only be administered once or twice to achieve long-lasting immunity. Others, especially vaccines that don't use the whole microorganism (subunit or acellular vaccines), are less potent, and several doses may need to be administered over a considerable time period, sometimes several years.
How effective is immunisation? It is important to recognise that the body's immune response is genetically determined. No matter how healthy a person is, they may respond very well to some vaccines but not to others. For example, one person may be fully immunised against measles after receiving the measles vaccine, but will not be fully immunised against mumps after receiving the mumps vaccine. Thus, in a given population, no vaccine will be 100 per cent effective: if a disease is prevalent in a community, some people who have been vaccinated will probably contract it. In Australia, the polio, measles and Haemophilus influenza type b vaccines protect about 95 per cent of those vaccinated; the diphtheria and whooping cough vaccines about 85 per cent; and the influenza virus vaccine about 70 per cent. The new acellular whooping cough vaccine may be slightly less effective than the traditional vaccine made from a killed, whole microorganism, but it gives fewer side-effects. A comprehensive vaccination program will limit the spread of a disease among a population, reducing the risk that non-immune people (ie, those people who have not been vaccinated and those who were vaccinated but whose immune systems did not respond to the vaccine) will become infected. This is known as ‘herd immunity’; eventually, when the number of non-immune people drops to a certain level, the disease will disappear from the population. For a highly infectious virus such as measles, about 95 per cent of people need to be immunised; for smallpox, which was less infectious, the figure was closer to 80 per cent.
Disease eradication A global vaccination program can result in the eradication of a disease that only affects humans. This is how smallpox was beaten. It was declared eradicated from the world in 1980 by the World Health Organization (Box 2). Following intensive immunisation programs, indigenous poliomyelitis was eliminated from the whole of the Americas in 1994, and the last case occurred in China in 1995. Indigenous measles has recently been eliminated from Finland and the Caribbean Islands, and in the Americas the only confirmed cases in 2003 were people who had contracted the disease elsewhere (Box 4: WHO's Global Programme for Vaccines and Immunization).
What are the risks? Like many other voluntary activities, such as driving cars or motor cycles, vaccination is not completely risk-free. The oral polio vaccine may cause poliomyelitis in about 1 in every million vaccinees; this is a tiny fraction of the number of healthy children who suffered (with many deaths) from this disease before vaccination was introduced. The traditional whooping cough vaccine causes some side-effects, most of which are minor. It has been accused of causing brain damage in about 1 in every 200,000 vaccinees but the causal relationship claimed was not accepted in a recent UK law case. In the UK in the late 1970s, the level of vaccination dropped from more than 80 per cent to 30 per cent following a media program about adverse reactions associated with this vaccine. Over the next 10 years there were two epidemics of whooping cough, each with some deaths and over 50,000 cases of the disease. By the early 1990s an intense vaccination campaign resulted in an immunisation rate of more than 90 per cent and there were very few cases of the disease reported. The use of the traditional whooping cough vaccine was suspended in Sweden in 1979 because of safety concerns. Many trials of different acellular vaccines were then carried out in Sweden in the ensuing years because of the resulting increased incidence of whooping cough. Now, many developed countries, including Sweden, the USA and Australia, have introduced acellular whooping cough vaccines to replace the traditional vaccine.
Vaccine safety has been examined in great detail in recent years by expert committees such as those convened by the US Institute of Medicine. Their findings show that the risks of side-effects are generally extraordinarily low.
Not immune to criticism Since Edward Jenner's experiment, vaccination has had its share of controversy. Different groups have protested and argued against this practice. Yet the weight of scientific evidence is overwhelmingly in favour of immunisation (Box 5: A controversial history). Parents should be aware that there are some rare complications that can result from vaccination. A far greater risk, though, will arise if the immunisation rate against these diseases continues to fall. It is only when immunisation rates are high that Australian children will be protected against polio, diphtheria, whooping cough or tetanus, diseases that only a few decades ago were greatly feared. We have already had a taste of low immunisation levels with the recent outbreak of whooping cough; we need a shot in the arm – or maybe several – if we’re to halt such diseases in their tracks. Boxes 1. Acquired immunity: The body's second line of defence 2. Smallpox – the eradication of a disease 3. The basics of making a vaccine 4. WHO's Global Programme for Vaccines and Immunization Related Nova topics: Bird flu – the pandemic clock is ticking Kissing the Epstein-Barr virus goodbye? Warmer and sicker? Global warming and human health
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Page updated June 2008.
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This topic is sponsored by The Walter and Eliza Hall Institute of Medical Research.
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