How immunization works?

Vaccination is an act of prevention by reproducing natural immune (response) mechanisms to fight a disease.

Immune response: responding to the threat of infection

The Immune response is a natural mechanism activated by the human body when it detects the presence of an infectious agent. It responds to the invasion by producing antibodies and competent cells in adequate quantities, targeting specifically the infection or the disease.1

... in two steps
During the first encounter with the infectious agent (the pathogen), the defense mechanism is not yet up and running, and the disease has time to develop. By the second encounter, however, the pathogen will immediately be recognized and eliminated before it has time to cause disease. This applies to measles or polio, as the body produces an immune response. Influenza is different, however, as it is a constantly mutating virus that avoids detection by our bodies’ surveillance system.

Infectious diseases

Infectious diseases are caused by microorganisms, such as bacteria, viruses, parasites, or microscopic fungi. They are a major cause of death, in particular in children below five.

Vaccination: stimulating the body’s natural defenses

Vaccination works using the body’s immune response.2 The vaccine creates a non-pathogenic alert, which triggers an immune response. When the body comes into contact with the infectious agent, it has been trained to defend itself on its own.

... without causing the illness
Vaccines undergo various processes in order to destroy their ability to cause disease while preserving their ability to stimulate an immune response. They are produced from bacteria or viruses, their components (polysaccharides, proteins), or the substances that they secrete (toxins). Vaccines achieve immune protection without having to contract the disease and suffer the potential consequences. For certain illnesses, several doses of a vaccine are required in order to achieve immunity and protection. Vaccination is typically recommended by the health authority of each country.

The principle of vaccination

Vaccination consists of introducing an agent (bacteria, virus, or molecule) into the body, which has no pathogenicity (the ability to make you sick) but still has immunogenicity (the ability to induce an immune response). Vaccines are manufactured with weakened or inactivated microorganisms or fragments of microorganisms.

The immune system: protecting our health

One of our immune system’s most important features is its ability to distinguish cells and molecules that belong to the body from those that don’t. Its role is to stop a pathogenic agent (a virus, bacteria, or parasite, etc.) from spreading inside our bodies.

...with two lines of defense:
  • The nonspecific or innate response, which consists of all the barriers and non-specific tools that prevent harmful substances from getting inside our bodies – such as our skin. It also includes non-specific cells (like macrophages) or molecules designed to eliminate pathogens or reduce their spread.
  • The specific response, which is also present from birth is infinitely more complex, timely and involves recognizing, destroying, and memorizing intruders, and neutralizing them if they reappear. The purpose of vaccination is to create this memory.
Immune system 


What is herd immunity?

  1. Collective immunity or herd immunity is achieved when a high percentage of a population has been vaccinated. In this case, the pathogens no longer are transmitted to the rest of the population. Thus, vaccinated individuals indirectly provide protection for vulnerable people who cannot get vaccinated.
  2. So vaccination is also an act of collective protection for certain contagious infections, since it prevents the illness from spreading.
  3. The vaccine coverage needed for herd immunity varies depending on the pathogen. For polio, for example, the rate of coverage needed is 70%. For whooping cough and measles, it can go up to 90-95%.

Immunization and eradication

Mass vaccination can stop the circulation of a virus to the point of totally eliminating it. In 1967, the World Health Organization set up a plan to fight against smallpox, and it was declared eradicated in 1980.3 On the same model, the World Initiative to Eradicate Polio was launched in 1988.4 In the 2020s, polio may well become the second human disease to have disappeared, thanks to vaccination.

1 - Centers for disease control and prevention CDC Understanding How Vaccines Work. Last accessed January 2018

2 - Stanley A. Plotkin. Correlates of protection Induced by Vaccination Clinical and Vaccine immunology July 2010 1055-1065 Last accessed January 2018

3 - WHO. Emergencies preparedness, response. Smallpox. Last accessed January 2018

4 - WHO. Poliomyelitis Fact sheet Updated April 2017. Last accessed January 2018