Vaccine

Manos de persona sanitaria sujetando un frasco con la vacuna y una jeringuilla

What is a vaccine?

A vaccine is a medical tool that generates immunity against a disease-causing pathogen. It “trains” the immune system to respond more quickly and effectively if it comes into contact with the real microorganism in the future, thereby preventing infection or significantly reducing its severity.

To achieve this, vaccines present the body with a safe version of the infectious agent or one of its characteristic parts. This can be achieved by using weakened or inactivated microorganisms, fragments of the pathogen, or even providing instructions for temporarily producing a small part of it. The objective is always the same: to simulate a first exposure without causing the disease. Vaccines are usually given by injection, although oral and nasal vaccines also exist.

How do vaccines work?

From an immunological point of view, vaccines work by introducing an antigen, i.e. a specific molecule from the pathogen (usually a protein or toxin), into the body, where it can be recognised by the immune system. In some vaccines, the antigen is administered directly; in others, genetic instructions are provided so that our own cells produce it temporarily.

The presence of the antigen activates the innate immune system, the body’s first line of defence, which in turn triggers adaptive immunity, a more specific response. This involves the activation of B lymphocytes, which produce specific antibodies against the antigen, and T lymphocytes, which coordinate the response and, in some cases, eliminate infected cells. This process neutralises the initial stimulus and generates memory T and B cells.

These memory cells remain in the body for long periods and can rapidly recognise the same antigen upon future exposure, reactivating the production of new antibodies. Consequently, when the actual pathogen infects the individual, the immune response is quicker and more effective, either preventing the disease from developing or reducing its symptoms.

Why are multiple doses sometimes needed?

For certain vaccines, particularly those that do not use live attenuated microorganisms, several doses are required to induce a robust and long-lasting immune response. These booster doses consolidate immunological memory and maintain protective levels of antibodies over time.

The importance of vaccination: herd immunity

Herd immunity, also known as community immunity,occurs when a large proportion of the population is immune, providing indirect protection against an infectious disease. Under these conditions, transmission of the pathogen becomes more difficult as there are fewer susceptible individuals to infect.

This protects people who cannot be vaccinated, such as individuals with immunodeficiencies (e.g. those living with HIV/AIDS or receiving immunosuppressive treatments), those with certain types of cancer, and those with severe allergies to vaccine components.

The percentage of the population that must be immunised to achieve herd immunity varies depending on how easily each disease is transmitted. For highly contagious diseases such as measles, it is estimated that around 95% of the population must be immune to prevent their spread.

How are vaccines developed?

The process begins with the identification of an antigen capable of inducing an immune response that can neutralise the pathogen. Before being tested on humans, the candidate vaccine is evaluated in preclinical studies using cellular and animal models to assess its safety and potential effectiveness.

If the results are promising, clinical trials in humans begin. Phase 1 involves administering the vaccine to a small group of healthy volunteers to assess its safety. Phase 2 involves increasing the number of participants to study the immune response in more detail and determine the appropriate dose. Finally, Phase 3 involves conducting a large-scale trial to confirm the vaccine’s effectiveness in preventing the disease and continue evaluating its safety in broader populations.

Once these phases have been successfully completed, regulatory agencies authorise its use if the benefits outweigh the risks. However, monitoring continues once the vaccine is administered to the general population (Phase 4), enabling rare adverse effects to be identified and the vaccine’s effectiveness in real-world conditions to be evaluated.

From years to months: how the development of the COVID-19 vaccine was accelerated

Vaccine development is usually a lengthy process, often taking up to 15 years due to the complexity of the research, clinical trials, and regulatory requirements involved. However, in the case of the pandemic caused by the SARS-CoV-2 virus, this process was extraordinarily accelerated, with the first vaccines becoming available in less than a year. This was made possible through unprecedented investment and international collaboration, over two decades of research into mRNA vaccines and viral vectors, and the parallel running of clinical trial phases — all without compromising on safety and efficacy controls.

Vaccines throughout history

The history of vaccines begins in the late 18th century, when Edward Jenner developed the first vaccine against smallpox by inoculating a person with the cowpox virus. This breakthrough paved the way for the eradication of the disease and marked the beginning of modern immunology.

In the 20th century, the development of new vaccines enabled serious diseases to be controlled worldwide. Notable examples include measles, a highly contagious infection responsible for millions of deaths before mass vaccination, and tetanus, a potentially fatal disease for which vaccination is crucial for preventing wound infections. Poliomyelitis is another significant disease, having been widespread in the first half of the 20th century and responsible for paralysing thousands of children.

While these diseases have been eliminated in many regions, they have not been eradicated and can reappear in areas where vaccination coverage declines, highlighting the importance of vaccination programmes.

Smallpox is the only human disease to have been eradicated globally, officially being declared so in 1980, and this was achieved through vaccination.