Covid-19: the mirage of collective immunity is still moving away a little
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.
Read alsoCovid-19: with the Delta variant, is “herd immunity” unattainable?
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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