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Herd immunity

If each disease has a different R0 value (eg measles with 12-18 and influenza 2-4) why is the level of herd immunity required constant at 95%

(18-1)/18= 94.4444%
(2-1)/2=50%

Reply 1

HarisMalik98

This isn't true, the theoretical herd immunity threshold is dependant on the R0 value.

The threshold is calculated roughly by:

1 - 1/R0.

Therefore, if disease 1 has a R0 value of 8. Then the herd immunity threshold would be 1 - 1/8 = 0.875 = 87.5%.

If disease 2 had a lower R0 at 3, then the threshold would be 1 - 1/3 = 0.667 = 66.7%.

These threshold values are only theoretical, however. There is no absolute guarantee you will stop transmission if you manage to pass the threshold. Theoretically, though, these values inform scientists how much vaccine coverage is required to ensure the best chance of breaking chains of transmission. Of course, having 95% immunity in the population will always be better than having a lower value closer to the threshold - and, is probably more desirable. But thresholds are not constant at 95% unless you have an R0 of ~20.

Reply 2

HarisMalik98

P.S ... Upon reading again, if you meant differences between different strains of the same species; then, like you said, the R0 can vary slightly depending on the strain and therefore so does the threshold value for that strain.

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