Vaccination is the administration of antigenic material (the vaccine) to stimulate an individual’s immune system to develop adaptive immunity to a particular pathogen. Herd immunity is a type of indirect protection from infectious disease that occurs when a sufficiently large percentage of a population has been vaccinated.
Vaccination has proven to be the most effective method of preventing infectious diseases, and widespread vaccination is largely responsible for the worldwide eradication of smallpox, and the elimination of serious conditions, such as measles, polio and tetanus in many areas around the world.
Active and passive immunity
Actively acquired immunity involves the development of an immune response either due to vaccination or natural exposure to a pathogen. Actively acquired immunity leads to long-lasting resistance to infection. A series of primary vaccinations with inactivated bacteria is usually required to induce an adequate immune response, and in most cases, boosters are required to sustain adequate immunity.
Passively acquired immunity does not involve a host immune response and occurs in one of two ways:
- Following a transfusion of immunoglobulins or;
- Following placental transfusion of maternal immunoglobulins in infants
Unlike actively acquired immunity, passively acquired immunity is relatively short-lived and does not lead to long-lasting resistance to infection.
Types of vaccine
There are four main types of vaccine that are currently in use:
- Live-attenuated vaccines
- Inactivated vaccines
- Subunit vaccines
- Toxoid vaccines
Live vaccines use an attenuated (weakened) form of the organism that is responsible for an infection. Live-attenuated vaccines are very similar to the naturally occurring disease that they help to prevent, and because of this, they tend to generate a strong and long-lasting immune response. Often just a small number of doses of a live-attenuated vaccine will result in lifelong immunity against a disease.
As live-attenuated vaccines contain a weakened form of the organism causing the disease, they can be dangerous in individuals with weakened immune systems and those taking immunosuppressive drugs.
Examples of live-attenuated vaccines include the following:
- Measles, mumps and rubella vaccines (MMR)
- Oral polio vaccine (OPV)
- Smallpox vaccine
- Yellow fever vaccine
- Chickenpox vaccine
- Rotavirus vaccine
Inactivated vaccines use a killed version of the organism that is responsible for the disease being vaccinated against. Because of this inactivation, the immunity provided is weaker than that provided by a live-attenuated vaccine. This means that several doses of the vaccine over a period of time (booster shots) are generally required for ongoing immunity to be generated.
Examples of inactivated vaccines include the following:
- Hepatitis A vaccine
- Inactivated polio vaccine (IPV)
- Rabies vaccine
Subunit vaccines use a very small part of an organism that has been selected for its ability to initiate a specific immune response, which is then isolated and purified. Because these vaccines use only a specific piece of the organism, they tend to generate a very strong immune response that is targeted against this key part of the organism.
They are usually safe to administer to people with weakened immune systems and chronic health problems, but, as with inactivated vaccines, booster shots are generally required for ongoing immunity to be generated.
Examples of subunit vaccines include the following:
- Haemophilus influenzae type b (Hib) vaccine
- Hepatitis B vaccine
- Human papillomavirus (HPV) vaccine
- Pneumococcal vaccine
- Meningococcal vaccine
Toxoid vaccines are made using a toxin that is produced by the organism causing the disease being vaccinated against. They generate antibodies that neutralise this toxin, as opposed to targeting the organism itself. Booster shots are often required to maintain immunity.
Examples of toxoid vaccines include the following:
- Diphtheria toxoid vaccine
- Tetanus toxoid (TT) vaccine
Contraindications to vaccines
All vaccines are contraindicated in individuals who have had:
- A confirmed anaphylactic reaction to a previous dose of a vaccine containing the same antigens
- A confirmed anaphylactic reaction to another component in the relevant vaccine, e.g. streptomycin or neomycin
Vaccination should also be avoided during times of acute febrile illness.
Vaccination in pregnancy and prematurity
There is no evidence of harm to pregnant women or foetus from vaccination with an inactivated, subunit or toxoid vaccine. Live attenuated vaccines, however, are contraindicated in pregnancy, but vaccination may be performed if the risks of infection exceed the risks of vaccination.
Premature infants, and also those with heart and lung diseases, should be vaccinated according to the usual immunisation schedule. Premature infants should be immunised at the normal times recommended for full-term babies, and there should be no correction for gestational age implemented.
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