The widespread use of vaccines globally has been highly effective in reducing the incidence of infectious diseases and their associated complications, including death. For centuries, infectious disease was the most common cause of disability and death worldwide, a situation that persists in the least developed countries of the world. Until the 19th century, it was unclear what caused these diseases and why some people became very ill with an infection while others were less seriously affected.

Based on research and observation, the medical community now knows that infectious diseases are caused by micro-organisms (pathogens). It is also now understood that the human immune system provides our defence against infectious diseases.

The immune system is made up of trillions of specialised cells (white blood cells) that detect and destroy pathogens or their toxins. Some white blood cells, which are known as lymphocytes, and the antibodies they produce, are highly specific. Each recognises only one pathogen or its toxin. A key feature of lymphocytes is that after an infection, lymphocytes specific to the pathogen will persist in the body. These specific, long-lived lymphocytes are called memory cells. If a person encounters the same pathogen again in the future, these memory cells will help the immune system mount a much quicker, larger and more sustained response that controls the infection more efficiently, often without development of any clinical symptoms.

The immune system’s capacity to have a memory of previous encounters with an infection is the basis for vaccination. Each vaccine contains one or more antigens from a pathogen (i.e. components able to stimulate immunity); types of antigenic material include the killed whole pathogen or components of it, or a live but weakened version of the whole pathogen. The antigens in a vaccine are recognised by lymphocytes and lead to development of memory cells as well as antibodies. If, after successful immunisation with a vaccine, a person is exposed to the actual pathogen, the memory cells enable the immune system to mount a rapid, sustained immune response, thereby greatly reducing the complications associated with a natural infection.

Immunisation with each vaccine protects an individual from a serious infectious disease and from associated long-term complications, which may include chronic organ damage and diseases such as cancer. Decreasing the number of people in the community who are infected with a particular pathogen has a positive impact on individuals who are susceptible to the infection because they are less likely to come into contact with it. This effect is called herd immunity. As a result, several infectious diseases have been controlled or almost eliminated in Australia, which would never have occurred just due to improvements in healthcare, sanitation or nutrition.

Many effective vaccines exist; nevertheless, researchers continue to develop new vaccines for use against infectious diseases for which no effective vaccines are currently available. A critical component of this vaccine development is effectiveness and safety testing. Before release for use in the broad community, a vaccine must undergo a series of rigorous clinical trials, each of which involves a greater number of participants. New and existing vaccines also undergo stringent monitoring once they are in widespread use in the community to ensure their ongoing safety and effectiveness.

Vaccines are the most successful form of disease prevention available, and will continue to be an essential tool in controlling infections and their complications. In the future, vaccines may also be effective in treating and preventing some non-infectious diseases.

This document aims to summarise and clarify the current understanding of the science of immunisation for non-specialist readers. The document is structured around six questions.

1. What is immunisation

The purpose of immunisation is to prevent people from acquiring infectious diseases and to protect them against the associated shortand longer-term complications. Immunisation describes the process whereby people are protected against an infection; vaccine refers to the material used for immunisation, while vaccination refers to the act of giving a vaccine to a person. Vaccines work by stimulating the body’s defence mechanisms (immune system) against an infection, helping it to detect and destroy the infection when it is encountered again in the future without development of significant symptoms or complications.

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2. What is in a vaccine?

Vaccines generally contain two main types of ingredients: antigens, which are designed to cause the immune system to produce a specific immune response; and adjuvants, which amplify the body’s immune response.

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3. Who benefits from vaccines?

In the short term, immunisation protects individuals from a specific infectious disease and its immediate complications. But immunisation may also have long-term protective effects—from cancer and other chronic conditions. An important feature of immunisation is that it also benefits the entire community. When a significant proportion of individuals in a community have become immune to a specific disease through immunisation, people who are still susceptible to the disease are less likely to come into contact with someone who is carrying the causative infectious agent.

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4. Are vaccines safe?

Vaccines, like other medicines, can have side effects, but the vaccines in current use in Australia provide benefits that greatly outweigh their risks. The great majority of reactions after vaccination are minor. Some adverse events coincide with vaccination but are not caused by the vaccine. Serious side effects from vaccines are extremely rare.

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5. How are vaccines shown to be safe?

Safety research and testing is an essential part of vaccine development and manufacture. Before vaccines are made available, clinical trials with increasing numbers of participants are required to study safety as well as effectiveness. After vaccines have been introduced into the community, safety monitoring continues.

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6. What does the future hold for vaccination?

In recent decades, vaccine technology has greatly improved, resulting in the production of better and safer vaccines against an increasing number of infectious diseases. The future of vaccination includes extending the use of existing vaccines, developing new technologies to deliver vaccines and generating new vaccines for both infectious and non-infectious diseases like cancer.

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© 2019 Australian Academy of Science