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Vaccines are biological products. In other words, they contain viruses, bacteria or parts thereof. They are administered in order to prevent infectious diseases in susceptible individuals. The majority are administered by injection, but some are administered orally or nasally.

Vaccines | PortalCLÍNIC

Vaccines operate by training the immune system to respond in the future to a virus or bacteria

Marta Aldea Epidemiologist

Vaccines can be classified into two main groups:

Inactivated vaccines

In these vaccines, the virus or bacteria we want to immunise against is alive (attenuated). Once inoculated, this micro-organism is able to replicate and stimulate the immune response of the recipient’s body. The virus or bacteria have been attenuated so that they cannot cause the disease being vaccinated against. The method most commonly used to attenuate the micro-organism is to pass (grow) the virus through cell cultures several times. Each time it is passed through the cell cultures, the virus loses its ability to replicate in human cells. Although this procedure means the virus loses its ability to replicate, it can still be recognised by the human immune system and will provoke an immune response. When inoculating a virus or whole bacteria, they have a powerful ability to provoke a good immune response. On the contrary, the amount of antigens we inoculate is large, and there is a greater likelihood of adverse reactions.

Inactivated vaccines

In these vaccines, the virus or bacteria has been inactivated, either using heat or other methods such as formaldehyde or formalin. This means that the micro-organism’s ability to replicate is inactivated, but its ability to stimulate the immune response is left intact. Losing the ability to replicate means that the capacity to generate an immune response is lower and has a shorter duration, meaning in general more doses are required to complete the vaccination schedule. It is also necessary to include adjuvants (substances that enhance the body's immune response) in the vaccine preparation in order to maintain long-term immunity. This kind of vaccine cannot cause disease under any circumstances.

There are different types:

Whole-cell: for example hepatitis A and influenza vaccines.

Toxoid: produced using detoxification of the toxin that causes the disease. These include the tetanus and diphtheria vaccines.

Subunit: only contain fragments of viruses, such as the flu vaccine.

Recombinant: made using the production of the antigen in non-human cells such as yeast, hepatitis B and papilloma cells.

Conjugate vaccines: such as pneumococcal, meningococcal, Haemophilus type B.

Vector vaccines: These vaccines use non-pathogenic viruses to present the genetic code of the micro-organism’s antigens to the recipient's cells so that they produce protein antigens that will stimulate the immune response. Vector vaccines are quick to produce and are also cheaper than nucleic acid vaccines. The carrier virus can have replicative capacity (Ebola vaccine) or non-replicative capacity (COVID-19 vaccine).

Nucleic acid vaccines (mRNA): the RNA strand carrying the virus antigen information is surrounded by a lipid nanoparticle that facilitates entry and gives it stability. Once inside the cell, this RNA transfers the information so that the cell's machinery can transcribe it and proceed to make the antigen that will stimulate the immune response. The RNA strand degrades and never integrates into the cell. COVID-19 is one example, but there will be more in the future.

DNA vaccines: are more stable and do not need to be protected by a lipid capsule. The DNA strand needs to be introduced into the cells. The technique used is called electroporation. It uses low-level electronic waves to allow the DNA to enter the cells and, once inside, the strand passes the information to its own messenger RNA and begins to produce the antigen. There are currently no DNA vaccines available on the market.