22 ms·
>Herd immunity isn't a binary thing. Moreso than you'd think. The effect of some percentage of the population being immune is to reduce the effective R0 (trans
by dTal 7y ago
>Herd immunity isn't a binary thing.
Moreso than you'd think. The effect of some percentage of the population being immune is to reduce the effective R0 (transmission ratio). At an R0 of less than 1, the disease cannot sustain community transmission and dies out. Above 1, you get exponential growth. This is a stark threshold. Usually you need a population that's 60-80% immune to achieve this threshold.
- cameronh90 7y agoTechnically, yes, there is an R0 above which a virus is self sustaining. But if something has R0 of 1.25 the herd immunity threshold is about 20%, whereas if it's 18, the HIT is nearly 95%. Additionally, R0 isn't an absolute property of the pathogen, so even assuming there is this absolute HIT, if R0 is around 1, some people will be above 1 and some below 1. As herd immunity increases, the R0 decreases. A disease with a low R0 spreads more slowly (hence the logistic curve) and is therefore easier to track and trace manually, and has a lower peak and total infection count. I don't think this is binary at all, or even _that_ stark of a threshold. Transmission doesn't continue at the same exponential rate until it hits around 60% and then suddenly disappear. It doesn't even come close to approximating that behaviour.