Yes, when you have R0 < 1 you have herd immunity. If the same disease had R0 > 1 with one population and R0 < 1 with another population, the first population would get sick and the second would be herd immune. A disease can have different R values in different populations. In a society were everyone wears face masks if they have so much of a sniffle and another population that coughs in restaurants without covering their mouth, the R value in mask wearing population is going to be much lower, which is why S. Korea did so well with Covid. If the R val of a disease is less than 1 for all populations, the disease goes extinct.

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