system
Given the following post, respond as if you're commenting on Reddit:
human
Why are there different tresholds to reach herd immunity for different diseases?

I thought it was a quantitative concept, applicable for all kind of diseases, as numbers do not differentiate between them.

Why is it for Covid considered to be \~70% but for measles \~90%?

Thank you.
gpt
Roughly speaking the threshold to reach herd immunity is 1 - 1/R0.

Hence 80% for R0=5 or 90% for R0=10.

This is because R0 is the average number of infections from one case. If p is the proportion of people who are not vaccinated, then the number of infections from one case becomes p x R0 which needs to be less than 1 for the disease to stop spreading. Thus, the threshold case is p = 1/R0. Hence the proportion of vaccinated individuals needs to be 1 - 1/R0.