Collective immunity makes it possible to put an end to a epidemic and is the percentage of a population that is immune to a disease. This group immunity can be achieved by allowing individuals to expose themselves to infection or by vaccination (if a vaccine exists). According to scientists, the vaccination rate should be around 70%, this is the level necessary for herd immunity.
How to break the transmission chain?
Collective immunity breaks the chain of transmission of a virus and for that, there are two ways to operate.
- By natural infection: the infectious agent is allowed to circulate until a certain percentage of people are infected. In theory, the more people who are infected, the more antibodies they develop against the disease and the less contagious they will be. Over time, the disease will eventually die out.
- By vaccination: vaccines are designed to protect people from harm pathogens (micro-organisms that can make you sick). The vaccine helps activate the immune system of the body to fight a group of pathogens or a certain pathogen. When we are vaccinated we are protected from infection and those who have not received the vaccine are likely to get sick. Herd immunity is a way to protect unvaccinated people by reducing the risk of them coming into contact with infection.
What is the threshold to be reached in order to put an end to an epidemic?
The number of people who must have specific antibodies for collective immunity is different for each disease, we cannot give a precise estimate. The calculation of this threshold depends on the basic reproduction number of the disease (R0), it is the average number of immunologically naive people that a subject will infect after contact. The higher the reproduction rate, the higher the percentage of immune individuals must be.
However, other parameters must be taken into account, the percentage to reach the collective immunity level may be revised downwards, in particular in the following cases:
- whether children under 10-12 years of age are less susceptible to infection;
- if the number of contacts with others varies enormously from person to person.
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