Vaccines

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.

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.

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.

Fig. 1 - Herd immunity

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.

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.

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.

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?