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1st screen- Disease with no vaccination First let’s look at how the disease spreads
in a naïve population, that is, a population that has never been exposed to the disease
pathogen (virus or bacterium) or to a vaccine.
The red circle represents the spread of the pathogen.
Each person who falls sick is shown in red.
Infectiousness is specific to each pathogen, virus or bacterium.
2nd screen - 50 percent coverage Now let’s look at the same situation, but
let’s assume that 50 percent of the same population is vaccinated. This is referred
to as 50 percent vaccination “coverage.”
A green circle denotes the people who were vaccinated.
The spread of the disease is completely different. All the people on the right are protected.
In the end, a certain percentage of the population is directly protected by the vaccine, as expected.
On top of that, a certain percentage is protected because it is not exposed to the disease.
It is the vaccinated people around this group who curb the spread of the pathogen.
This additional protection is called “herd immunity.”
The individuals protected in this manner are shown here in purple.
3rd screen - 80 percent coverage In this third scenario, the vaccination coverage
rate now rises to 80 percent.
The spread of the disease is contained very quickly. Herd immunity is in full force, and
a large percentage of the population is protected, whether directly or indirectly.
This is the rationale for high vaccination coverage. In fact, when this occurs, we protect
not only the individuals the health care system can reach, but also those who could not be vaccinated.
This is why vaccines are integral to public health policies.