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A vaccine is an antigen-containing substance that is introduced into the body with the intention of stimulating an immune response against a particular disease. When a person receives a vaccine they become vaccinated. Vaccines contain weakened (live attenuated) or dead (inactivated) versions of a particular disease-causing pathogen. As the pathogen has been weakened, most people will experience no symptoms of illness from a vaccine.

Vaccination is an example of artificial active immunity, where an immune response is induced in an individual without them suffering any symptoms of the disease.

How do vaccines work?

The weakened pathogen is introduced into the individual either via injection or orally.

Usually, vaccines are injected because often introducing a weakened pathogen orally will lead to it being broken down in the stomach.

Following vaccination, the primary immune response is triggered in response to the foreign antigens. Here, T Helper cells trigger B plasma cells to secrete specific antibodies against the pathogen's antigens.

This primary response also leads to the production of memory cells. Memory cells provide long-term immunity as memory cells will quickly produce specific antibodies in a secondary response if the same pathogen is encountered again. This vaccinated individuals from suffering from prevents symptoms of a disease if they are infected. For some diseases, booster vaccines (secondary or tertiary vaccines) are administered to make sure memory cells are still present in the body.

Examples of vaccines

The measles, mumps and rubella (MMR) vaccine is given to most children during infancy in two doses, at least 4 weeks apart. This vaccination protects children against measles, mumps, and rubella on a long-term basis.

The HPV vaccine is given to children in the UK between ages 12 and 13. The HPV vaccine protects against the human papillomavirus, a name given to a group of very common viruses. High-risk types of HPV are associated with the development of cervical cancer, vaginal cancer, vulval cancer, anal cancer, cancer of the penis and some cancers of the head and neck. This vaccination, therefore, protects against types of cancer and the associated risks from contracting HPV.

Types of vaccine

There are several types of vaccines including attenuated pathogens, inactivated vaccines and mRNA vaccines (not widely used) which show promise for the future.

Live attenuated vaccines

These vaccines contain whole pathogens that have been weakened. Weakened pathogens multiply slowly, which allows the body to recognize the antigens on the pathogen and produce a primary immune response without becoming overwhelmed and showing symptoms of the disease.

This vaccine can be unsuitable for those with weak immune systems as their primary immune response is slower and the pathogen causes symptoms before antibodies can be produced. Life attenuated vaccines tend to produce longer-lasting and stronger immune responses. Examples of live attenuated vaccines include the MMR vaccine and the vaccine for yellow fever.

Inactivated vaccines

These vaccines contain whole pathogens that have been killed or subunits of the pathogens, such as proteins.

These vaccines are more suitable for those with weak immune systems because they do not contain living pathogens, so there is no risk of causing disease. Adjuvants, a substance that enhances the body's immune response to an antigen, may be added to these vaccines. These may cause local allergic reactions, such as a sore arm, in some individuals.

mRNA vaccines

mRNA vaccines are a new type of vaccine that show promise for future vaccines against previously difficult to target diseases, such as HIV.

mRNA vaccines do not use the complete pathogen but rather deliver only its genetic code, mRNA. This mRNA teaches the body's cells to produce antigen proteins specific to the virus, which can then be presented on the surface of these cells, triggering an immune response. The first viable forms of the vaccine are famously known due to their use in the coronavirus (COVID-19) pandemic.

Table 1. An overview of vaccine delivery methods.

Delivery method




Live attenuated

Often results in longer immunity due to the use of the whole pathogen.

Not suitable for immunocompromised people.

MMR vaccine

Must be refrigerated at all times.

Yellow fever vaccine


Most common form today.

Localised allergies are still possible.

Rabies vaccine

Lower chance of allergic response.

Booster vaccines are more likely to be required

Hepatitis A vaccine

Better for immunocompromised.

Herd Immunity

Herd immunity is achieved when a sufficient proportion of the population has been vaccinated to break the chain of a pathogen's transmission, making it difficult for that pathogen to spread through the population and infect vulnerable groups who are unable to receive a vaccine. Pathogens are typically transmitted through close contact between individuals, therefore if most of the population is immune and cannot be infected, then it is very unlikely that a susceptible person who is unvaccinated will come into contact with a carrier of the disease.

Herd immunity, therefore, helps to protect individuals who cannot be vaccinated. Babies and the very young cannot be vaccinated because their immune systems are still underdeveloped and not yet fully functional, putting them at risk of serious illness if exposed to even a weaker form of the vaccine pathogen. Those who are ill or have compromised immune systems due to autoimmune diseases or immunosuppressant drugs also cannot be vaccinated due to the medical risks it poses.

Vaccines herd immunity StudySmarterFigure 1. Herd immunity. Source: freepik.

How is a successful vaccination program run?

In short, the criteria are:

  1. Sufficient quantity produced.
  2. Economically viable.
  3. WIdely available.
  4. Few or no side effects.
  5. Suitable ways for producing, storing and transporting the vaccine.
  6. Access to technologically advanced equipment.

Economical viability

The vaccine must be economically available. In other words, it must cost a reasonable amount of money to research, produce, and deliver the vaccine. If the vaccine is too expensive to produce there is a risk that not enough will be made to vaccinate enough people to produce herd immunity. In addition, the vaccine must also be made economically available to the population, people should be able to afford the vaccine if it isn't free and must be able to afford associated costs such as transport to the vaccine centre.

Side effects

Vaccine developers should also work to ensure that there are few, if any, side effects from the vaccine. This is because fewer people will be likely to get the vaccine if they know that they will experience unpleasant side effects from it.

Suitable equipment and handling available

There must also be suitable ways of producing, storing, and transporting the vaccine. Researchers should have access to technologically advanced equipment and there should be hygienic conditions to store the vaccine in. Many vaccines must be refrigerated or kept cool, so vaccine centres should have access to fridges and freezers at the correct temperatures to keep the vaccine good for use.

Problems with vaccines

It is important to note that vaccines are not a total 'cure-all' and often do not entirely eradicate a disease from the population. Many also have ethical oppositions to the way vaccines are manufactured, as well as how they are used. These issues must be taken into account when considering the success of vaccines to combat diseases in populations.

Why vaccines are not completely foolproof"?

Vaccines may fail to induce immunity in some vaccinated individuals. For example, if a person has a defective immune system and is vaccinated, their immune system is unlikely to properly respond to the vaccine and produce a successful long term immune response.

Individuals may also develop the disease immediately after vaccination, where the immune response to the vaccine has not been fully developed and memory cells have not yet been produced. This allows for the person to become ill from the disease, while also harbouring the harmful pathogen and spreading it to others.

Reasons why people might avoid getting vaccinated

Individuals may choose not to be vaccinated for religious, ethical, medical, or political reasons.

For example, during the coronavirus pandemic, many individuals refused to be vaccinated due to ideological beliefs. Misinformation around vaccines, such as that they cause autism, and conspiracies such as that vaccines contain government tracking devices, can seriously damage the effectiveness of community vaccination programs and undermine trust in medical professionals.

Pathogenic mutations

The pathogen being vaccinated against may mutate rapidly, suddenly changing its antigens. This means that vaccinated individuals can still be infected because the new pathogen antibodies will be different from the antigens on the weakened pathogen used for immunisation.

The memory cells will be unable to recognise the mutated pathogen. Antibodies, therefore, will not be produced against the mutated pathogen and the person will become infected. This is called antigenic variability.

An example of this is the mutation of the influenza virus, which changes its antibodies so often that every year researchers must synthesize a new flu vaccine based on current emerging strains of the virus so that they can vaccinate vulnerable groups, such as the elderly. This is called a continuous vaccine trial.

Due to antigenic variability, there may be so many varieties of the pathogen that it is practically impossible to vaccinate against them all or to develop a single vaccine effective against each mutated strain. This can be seen with the common cold virus, which has over 100 varieties and is still evolving all the time.

Vaccines mutations in influenza virus StudySmarterFigure 2. Mutations in the influenza virus. Source: Umberto NURS, CC BY-SA 4.0, via Wikimedia Commons.

Pathogens can hide from the blood cells

Pathogens may also be able to escape the body's immune system through 'hiding' from white blood cells. This may occur by pathogens concealing themselves within cells, a common technique for viruses, or in other places out of reach of defense mechanisms. For example, cholera bacteria occupy the intestines. This is called antigenic concealment.

Vaccines and ethical issues

Ethical issues include animal testing, human trials, monetary issues and others.

Animal trials

The development of new vaccines often involves the use of animals for testing before clinical trials progress to testing new vaccines on humans. Some question the ethics of animal testing as animals cannot consent to be tested. In addition, trial vaccinations often have unknown and potentially harmful side effects.

Human trials

The issue of vaccine trials also extends to humans, however, with questions being raised about whom vaccines should be tested on and how much risk it is acceptable to ask those participating in trials to take on in the interests of public health and the collective benefit. There is also the question of who should research and manufacture vaccines.

Some vaccines also have side effects which can sometimes cause long term harm depending on the reaction of the person being immunised. Researchers must balance the risk of side effects against the risks of developing the disease being vaccinated against. They must also consider which of these risks is likely to cause the most harm to individuals. The individual health risks from being vaccinated must also be balanced against the advantages for the larger population if the disease is able to be successfully controlled as a result of mass vaccination.

Should some vaccines be compulsory?

Ethical issues have also been raised as to whether vaccination should be compulsory or not. For a vaccination program to be successful the majority of the population should be vaccinated. Therefore, it would arguably be in the best interests of the whole population if vaccination was compulsory for healthy individuals. However, this conflicts with people's free will to choose for themselves what they want to do.

During the coronavirus pandemic, this has been a topic of debate, with people asking if it is acceptable to restrict those who are unvaccinated from accessing certain venues and events due to the increased risk of transmission that they pose.

Prioritising who should be vaccinated

When vaccine stocks are running low or when a vaccine is very expensive to manufacture, who should be prioritised for vaccination?

Monetary issues

Monetary issues must also be taken into consideration. Is it right to charge money for vaccinations or should they be free? How will vaccination programs be funded? How is this funding balanced with the need to fund treatments for other diseases?

Vaccines - Key takeaways

  • Vaccines are a preventative measure against a person contracting a disease caused by a pathogen.
  • Vaccines contain a dead or weakened version of a pathogen did triggers of immune response When Introduced into the body but does not cause symptoms of the disease being vaccinated against. Vaccines trigger memory cells to develop and produce long term immunity.
  • There are two types of vaccines: live attenuated vaccines and inactivated vaccines.
  • Herd immunity is when enough of the population has been vaccinated did the virus is no longer transmitted between the population. Herd immunity protects vulnerable populations who cannot be vaccinated.
  • There are ethical issues surrounding the use of animal testing in vaccine production. There are also ethical questions as to whether vaccination should be mandatory and how to balance the personal risk of side effects with the need to protect the larger population from disease.


Vaccines prevent disease in two ways. Vaccines prevent disease on an individual level by immunizing individuals against disease. When a person is vaccinated they are then immune to the disease vaccinated against. Vaccines also prevent disease on a wider community level. If the majority of the population is vaccinated then the spread of disease is stopped and herd immunity is reached. This is when unvaccinated individuals are also protected from disease because enough people have been vaccinated to break the diseases' chain of transmission.

Vaccines contain antigens from a specific disease that is being vaccinated against. Live attenuated vaccines contain weakened or damaged versions of pathogens, while inactivated vaccines contain whole pathogens that have been killed or subunits of the pathogens, such as proteins.

Adjuvants are added to vaccines because they promote the uptake of the antigen by the immune system. They help to increase the body's immune response to a vaccine.

In research, questions as to who should research and manufacture vaccines must be raised, as well as what sort of profit they should be able to make from manufacturing vaccines. Vaccines are also often tested on animals in clinical trials, which some see as unethical because animals are unable to consent to testing. Ethical issues arise as to who should be vaccinated and if vaccines should be compulsory - how do you balance the need for personal free will with the need to protect the wider community from disease?

The antigens in vaccines stimulate a primary immune response. Helper T cells stimulate B cells to secrete specific antibodies against the vaccine's antigens. Memory cells are then produced which provide immunity as they deliver a fast secondary response if the pathogen is encountered in the body again.

Final Vaccines Quiz


What is inside a vaccine?

Show answer


Inside a vaccine is the specific antigen of the pathogen being vaccinated against. Vaccines contain either a weakened or dead version of these pathogens to allow the immune response to easily overcome the infection and prevent the vaccinated individual from developing symptoms of the disease.

Show question


How do vaccines work?

Show answer


Vaccines work by triggering the primary immune response. When the antigen from the vaccine is detected in the body T cells are stimulated. Helper T cells stimulate B cells which secrete specific antibodies against the pathogen. Memory cells are also produced. These provide long-term immunity because they quickly secrete specific antibodies and destroy any of the same pathogens that enter the body in the future.

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What type of immunity does vaccination produce?

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Vaccination is an example of artificial active immunity. It is artificial because antigens are artificially introduced by humans rather than naturally through infection. It is active because the immune response develops antibodies using B cells, rather than antibodies being directly introduced into the body, which is passive immunity.

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What are the two different types of vaccines?

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Live attenuated and inactivated are the two different types of vaccines. Live attenuated vaccines contain a weakened or damaged version of a pathogen. Inactivated vaccines contain a dead version of the pathogen or a subunit of the pathogen, such as proteins.

Show question


What is an example of a vaccine?

Show answer


MMR, the measles, mumps and rubella vaccine OR the HPV vaccine

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What is herd immunity?

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Herd immunity is when the majority of the population are vaccinated against a disease, resulting in a break in a diseases' transmission. This prevents those who are unvaccinated from coming into contact with the pathogen and becoming ill.

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Who does herd immunity protect?

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Herd immunity protects those who are unable to get vaccinated, such as the very young and very old, and those who are immunocompromised.

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How is herd immunity achieved?

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Herd immunity is achieved by vaccinating the majority of the population. This level of vaccination is achieved through the execution of a successful vaccination program in communities or countries.

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Why might a vaccinated individual still become ill with the disease they were vaccinated against?

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A person may have a weak or damaged immune system and so become ill from the vaccine (a live attenuated vaccine) because their immune system is unable to properly respond to the weakened pathogens. A person may also contract the illness immediately after being vaccinated and so become ill because their boy has not yet produced memory cells and the primary immune response is too slow.

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What is antigenic concealment?

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Antigenic concealment is when pathogens 'hide' from the body's immune system by concealing its antigens from detection. This may occur through pathogens hiding within host cells or by living in places out of reach of the immune system, such as the intestines.

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What is antigenic variability?

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Antigenic variability is when antigens on the cell surface of a pathogen frequently mutate and change shape. This means that if memory cells are produced against a specific antigen and antigen variability occurs there will be no immune response because the new antigens will not be recognized.

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Define a vaccine.

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A vaccine is an antigen-containing substance that is introduced into the body with the intention of stimulating an immune response against a particular disease.

Show question


What is an inactivated vaccine?

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A vaccine that contains whole pathogens that have been killed or subunits of the pathogens, such as proteins. 

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What is an mRNA vaccine?

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mRNA vaccines deliver a specific genetic code from the pathogen.

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Vaccines are only available for viral infections. True or False?

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