What is immunisation?
Immunisation is a way of preventing infectious diseases through the use of vaccines.
How does the immune system fight infection?
When a pathogen (such as a bacterium or virus) enters the body, the immune system recognises that the pathogen is foreign and prepares to fight the intruding organism using antibodies. If the pathogen is one that the immune system has not come across before, the immune system has to make specific antibodies that can inactivate the pathogen. Antibodies are protein molecules that have specialised receptors (binding sites) which attach to structures (antigens) on the infecting micro-organisms. This leads to the inactivation of the pathogen, which is then prevented from causing any further damage to the host (person or animal).
After recovery from an infection, our body keeps a memory of the specific antibodies that were produced during the infection. If the same pathogen is met again, then the same sort of antibodies can quickly be produced and the pathogen gets destroyed, mostly before illness occurs – much quicker than the first time it was encountered. This is why we are usually immune long term to an illness, such as chickenpox and mumps, after having had such infections.
How does immunisation work?
Immunity is the state of protection that results from active or passive experience with foreign antigens. Immunisation is performed to induce an immune response (passive immunity) and mimics the response that would occur had we had an infection instead (active immunity).
Active immunity is when a person develops an immune (antibody) response by natural exposure to organisms or other immunising products.
Passive immunity involves the use of vaccines which are made to protect against bacteria or viruses that cause infection. Vaccines contain weakened or dead pathogens or, more commonly, components of pathogens. These components, known as antigens, do not cause disease but are able to stimulate the immune system into making antibodies in the same way that live pathogens induce an immune response.
Protection from disease through vaccination (passive immunity) does not happen immediately after an injection has been given. It takes time for the body to produce antibodies to the components in the vaccine and it may take a number of injections given at specified time intervals to provide adequate protection from disease. For instance, with the influenza vaccine, it might take the body up to two weeks to create enough antibodies to be protected from influenza infection.
Antibodies do not permanently stay circulating in the blood but once immunised, either naturally or by vaccination, if stimulated the immune system in your body has memory and is able to reproduce more antibodies. Commonly, more than one dose of a vaccine is given over a period of time. The first dose ‘primes’ the immune system, then subsequent doses help increase the immune response; ie the numbers of antibodies circulating in the blood. Sometimes, a ‘booster’ dose is given months or years after vaccination. This is to reproduce more antibodies and to extend the protection long term; for example, when a person needs a tetanus injection after standing on a nail. This is because once the antibodies have disappeared from circulation in the body, that person is susceptible to the illness again. A booster dose is used to stimulate the memory cells to produce more antibodies and thus prevent infection.
Immunisation versus vaccination – what is the difference?
The terms ‘immunisation’and ‘vaccination’ are often used interchangeably but they have subtly different meanings. Vaccination refers to the process of administering a vaccine, usually through a syringe. Vaccination is intended to induce immunity. Immunisation is the process by which immunity is developed following a vaccine.
How are vaccines made?
There are two types of vaccines that produce immunity: live vaccines and vaccines containing dead organisms or components of dead organisms.
Live vaccines use weakened, but live, pathogens. In the laboratory, pathogens are weakened or modified so that they can no longer produce disease. This is known as attenuation. Attenuated pathogens can still stimulate the immune system, leading to immunity. Live vaccines are very effective. An example of a live attenuated vaccine is the measles, mumps and rubella (MMR) vaccine.
Inactivated vaccines use entire pathogens that have been inactivated with chemicals or heat, or just selected parts of pathogens. For example, toxoid vaccines are made from the toxins that some pathogens produce. These are altered so they are no longer toxic, but still produce an immune response. Subunit vaccines are made from particular antigenic parts of pathogens, which have been purified away from the entire pathogen. Recombinant vaccines are made from yeast cells that have had some of the genome from a pathogen inserted into them. And conjugated vaccines are made from parts of pathogens that have been chemically joined to other substances that the immune system can easily recognise, such as specific proteins. An example of an inactivated vaccine is the hepatitis B vaccine.
Vaccines also contain other components, such as inactivating agents, preservatives, stabilisers and adjuvants.
Passive immunity vaccines are made by pooling serum from many individuals who are immune to the disease of interest, and purifying the antibodies. An example of a passive immunity vaccine is tetanus immunoglobulin, which can be given if a wound is sustained that is considered to pose a risk of tetanus infection. This is different to the tetanus vaccination that is part of the regular vaccination schedule, which is an active immunity vaccine and provides longer-term immunity.
Which vaccines are available in New Zealand?
The National Immunisation Schedule funds vaccinations for a variety of diseases, including diphtheria, tetanus, acellular pertussis, polio, Haemophilus influenzae type B, hepatitis B, meningococcal B, measles, mumps and rubella. Vaccinations start at six weeks of age and continue on a regular schedule from then. Influenza vaccinations are also available free to those over 65 and with certain health conditions, and tuberculosis and pneumococcal disease vaccines are available to people who are considered at risk. The programme is reviewed every two years.
Other vaccinations are also available that are not publicly funded, but recommended in certain situations, such as when travelling to certain countries or for people who have weakened immune systems. An example is hepatitis A vaccine.
How effective are vaccines?
The effectiveness of vaccines varies but most are extremely effective. Polio, measles, mumps and rubella vaccinations, for example, are all more than 90% effective.
Are vaccinations safe?
There is quite a high level of concern in the general public about vaccine safety, and there is much coverage in the media and on the Internet.
A range of minor reactions are common after vaccinations. These include localised redness, swelling and pain at the injection site, and ‘whole body’ responses such as fever, nausea, tiredness, headache and aches and pains. Live vaccines cause a mild infection, so can cause mild symptoms of the disease.
Rarely, some people may experience more serious adverse events following a vaccination. Careful analysis is needed to make sure there is a cause-and-effect relationship between a vaccination and any adverse event. In order to facilitate this, all serious unexpected events must be reported to New Zealand’s Centre for Adverse Reactions Monitoring (CARM), which carries out an assessment to see if the event is vaccine-linked. Similar monitoring is also carried out overseas.
Some serious events have been shown to be caused by particular vaccines. For example, injected vaccines carry a small risk of a serious allergic reaction, and there has been shown to be a causal link between the rubella vaccine and acute arthritis (although the risk is very small and the effect uncommon).
Most investigations show that there is no causal link between a serious event and a vaccine. For example, suggestions of a possible link between the MMR vaccine and autism have received extensive media coverage. Studies, however, have shown that there is no evidence of a link between the vaccine and autism.
The risk of contracting a disease is usually significantly higher than the risk of an adverse effect from a vaccination. Also, the higher the rate of vaccination in a population, the lower the risk of a disease outbreak. This is known as herd immunity.
There is a variety of measures in place to help make sure vaccines are as safe as possible. For example, to gain approval for use, all vaccinations must be thoroughly tested, and each new batch is also tested before distribution.
Ministry of Health. 2006. Immunisation Handbook 2006. Wellington: Ministry of Health.
Baxter, D (2007). Active and passive immunity, vaccine types, excipients and licensing. Occupational medicine 57, 552-556.
Serpell, L. and Green, J. (2006). Parental decision-making in childhood vaccination. Vaccine 24, 4041-4046.
This Science Byte was reviewed by Dr Diana Martin, Principal Scientist – Clinical Microbiology, ESR (Environmental Science and Research)