DECRYPTION – Not only do vaccines lose effectiveness over time, the Delta variant hasn’t helped matters.

This is characteristic of mirages: the closer you get to them, the further they go. The same is true today of collective immunity. While vaccination can accelerate the protection of a growing part of the population, it unfortunately seems that we will never be able to reach the level required for the epidemic to end on its own.

At least partially, the fault lies with the Delta variant. In fact, the more contagious a virus, the higher the threshold required to achieve collective immunity. This corresponds to a simple mathematical formula: 100 × (1-1 / R0) where R0 is the basic reproduction rate of the virus (i.e. the average number of people infected by an infected person in a population that has never encountered virus in the absence of a control measure). With the initial virus appearing in China, R0 was worth 3, so it was enough for 66% of the population to be perfectly immune to “extinguish” the epidemic. This figure rose to 75% with the Alpha variant

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