- The body's immune system “remembers” an infectious agent to which it has been exposed, so that if that agent is encountered again, the immune system response will be more efficient.
- A vaccine is a substance given to a person in order to induce immunity to a specific infectious disease.
- A vaccine consists of a weakened strain of the infectious organism, killed organisms, or a part of the organism such as a weakened form of toxin that it produces.
- Vaccines are one of the most important strategies available to help prevent infectious diseases.
- Some people are concerned about vaccine side-effects, but these are usually mild and a small price to pay for immunity to a potentially fatal disease.
- A large number of vaccines are available to help prevent viral and bacterial infections, such as influenza and tuberculosis.
How do vaccines work?
In order to understand how vaccines work, it is useful to understand a little about how your body’s immune system works. When you are first exposed to an infectious agent (such as the influenza virus, or the bacteria that cause pneumonia or typhoid), your body’s immune system recognises that certain substances (often a protein or part of a protein) on the surface of the organism are foreign. These foreign substances are called “antigens”. As part of the immune response, proteins (called antibodies) are produced by white blood cells. These antibodies are secreted into the bloodstream and are able to bind to antigens, and either neutralise them entirely or allow other white blood cells to destroy the organism more rapidly. In addition, some white blood cells are able to recognise a foreign antigen directly and allow the immune system to destroy that foreign antigen.
What is interesting is that once the immune system has been exposed to a particular antigen, it “remembers” that antigen, so that if the antigen is encountered again, the immune system response will be much more rapid and efficient. Antibodies to that antigen may already be present in the blood circulation, so the antigen can be neutralised almost immediately. The white blood cells that recognise the antigen will also respond more rapidly. This is why you are usually immune to some infections (such as measles, mumps and German measles) if you have had them previously. Of course, the organisms that cause disease have also evolved mechanisms to avoid the immune system, so not all infections confer protective immunity. Some bacteria and viruses have a large number of different strains, each of which has different antigens. Thus you could be re-infected by a different strain of the organism, and because the antigens are different, you could fall ill again. This is particularly true of the viruses that cause the common cold.
So, let’s get back to the original question of how vaccines work!
A vaccine is a substance given to a person in order to induce resistance to a specific infectious disease. The vaccine contains either a part or all of the organism for which it is intended to confer immunity (i.e. it contains some or all of that organism’s antigens). Alternatively, a vaccine can consist of a weakened form of a toxin that the organism produces (this is called a toxoid). When you are vaccinated, your body’s immune system is exposed to the antigens, and makes antibodies and develops white blood cells that will remember the antigen if it is encountered again. If you are then exposed to that particular organism or toxin, your immune system will be able to mount a much better response, which will either prevent you getting the infection entirely, or will make the symptoms of the infection much milder than they would have been otherwise.
What is contained in a vaccine?
There are essentially three different types of vaccines currently in use:
These vaccines contain weakened strains of the organism. The organism in the vaccine multiplies in the body, but not enough to make one sick. The antigens on the organism stimulate the immune system to make antibodies.
- The vaccine for tuberculosis
- Measles vaccine and the MMR vaccine
- Oral polio vaccine
The organism in the vaccine has been killed in such a way as to preserve the antigen structure. Thiu, although the organism cannot multiply in the body, the antigens on the dead organism are still able to stimulate immunity.
- Typhoid vaccine
- Pertussis (whooping cough) vaccine
- Hepatitis B vaccine
As the name implies, the vaccine does not contain the whole organism, only a part of it. Certain antigens are very good at stimulating immunity, while other antigens are not particularly good at this. Ideally, subunit vaccines contain only antigens that are very good at stimulating immunity. The antigens in a subunit vaccine can either be a part of the organism itself (e.g. part of the cell wall), or a toxin the organism produces that is responsible for many of the symptoms of infection. The immune system would thus be able to neutralise the toxin on re-exposure (by means of antibodies), and thus limit the amount of damage done by the infection.
- Haemophilus influenzae vaccine (contains part of the cell wall)
- Diphtheria vaccine – contains an inactivated toxin.
Why should I get vaccinated?
Vaccines are one of the most important strategies available to help prevent infectious diseases. In the past, before vaccines, many children died or had severe complications following infections. Vaccines have helped to eliminate certain diseases (such as smallpox) and make others very uncommon (such as measles and polio). The more people are vaccinated, the more successful the vaccination programme will be, since there will be fewer people available for the organism to infect. When fewer people are infected, there will be less transmission of the organism, and hopefully the disease can be eradicated. This obviously only holds true for organisms that affect humans exclusively.
Some organisms that cause infection can be found in various wild or domestic animals as well (sometimes without causing disease in these animals). In such cases it would be much more difficult to eradicate the organism completely, but vaccination would still prevent humans from getting the disease.
Why do I need more than one dose of a vaccine, and why do I need boosters?
Because many vaccines are either dead organisms, or contain only part of the organism, the immune response is not as vigorous as if you’d been infected with the live, pathogenic (disease-causing) strain. You thus need three or four exposures to the vaccine to allow your immune system to develop a properly protective response. Boosters are given for similar reasons. Often five to 10 years after your initial vaccination, your immune system will have decreased the level of circulating antibodies to levels that are not really protective. By re-exposing the immune system in the form of a booster, it will be stimulated to produce more antibodies, and more white cells that can “remember” and recognise the antigen. In some situations, the immunity does last for life and you don’t need boosters. Sometimes, by the time 10 years have passed, you are at little risk of getting the infection anyway and thus boosters aren’t given.
Are vaccines safe?
Some people are concerned about side-effects of vaccines. It is true that some vaccines have side-effects. These are usually mild – headaches, sometimes muscle aches and fevers – and usually only last a few hours. This is actually quite a small price to pay for being immune to a disease that could possibly kill you, and at the very least will make you feel much worse than the side-effects did.
Some vaccines, notably the pertussis vaccine, have reportedly caused more severe side-effects, such as fever, seizures, and possibly brain damage. It is important to note, however, that there is some debate as to whether certain side-effects (especially chronic nervous system disorders) are actually due to the vaccine; that if the side effect is due to the vaccine, it is still extremely uncommon; and that if you are not vaccinated, you probably stand a better chance of getting the disease than you did of getting the side-effects. In addition, newer vaccines to pertussis are being developed, and these should have even fewer side-effects than the one currently in use.
Which vaccines are available?
A large number of vaccines are available, to prevent both viral and bacterial infections. Some of these vaccines are included in the Department of Health’s immunisation programme, while others are available for specific people or circumstances.
Routine immunisation schedule in South Africa
- At birth (OPV, BCG)
- 6 weeks (OPV, DTP, HBV, Hib)
- 10 weeks (OPV, DTP, HBV, Hib)
- 14 weeks (OPV, DTP, HBV, Hib)
- 9 months (Measles)
- 18 months (OPV, DTP, Measles)
- 5 years (OPV, DT)
BCG = bacillus Calmette-Guérin (tuberculosis) vaccine
OPV = oral polio vaccine
DTP = diphtheria, tetanus, pertussis (whooping cough) vaccine
DT = diphtheria, tetanus vaccine
HBV = hepatitis B vaccine
Hib = Haemophilus influenza group b (meningitis) vaccine
These vaccines have been grouped together so that they can be conveniently received at regular intervals. The different vaccines do not interfere with one another and there is no increased risk of serious side-effects when they are given at the same time.
Reviewed May 2007 by Dr Diana Hardie, Clinical Virologist, employed jointly by University of Cape Town and the National Health Laboratory Service (NHLS)
Written by Dr Andrew Whitelaw, MBBCh (Witwatersrand), MSc (UCT), FCPath (Micro) (SA), Clinical Microbiologist, Department of Microbiology, University of Cape Town and Groote Schuur Hospital.