Regarding the positivity rate in India stalling out, I would be hesitant to over-interpret this. There is a natural limit to the positivity rate somewhere below 100% (it might even be roughly 25% as we’re currently seeing).
A simple model as an intuition-pump: there are two subgroups. Group 1 would like a test, but will elastically respond to any extra hassle of getting one. Group 2 absolutely needs a test and will not desist, e.g., they strongly believe they have Covid, and without a test they cannot get access to medical treatment. As tests get scarce, group 1 will gradually drop away and test takers converge to being drawn 100% from group 2. But those test-takers will obviously have lower than 100% positivity rate: if this wasn’t the case, we wouldn’t need tests at all! There are other diseases and circumstances that cause people to join group 2.
In a nutshell, positivity rate is only a proxy for undiagnosed Covid cases within some unknown operating envelope (I’m guessing 5-20%?).
Regarding the positivity rate in India stalling out, I would be hesitant to over-interpret this. There is a natural limit to the positivity rate somewhere below 100% (it might even be roughly 25% as we’re currently seeing).
A simple model as an intuition-pump: there are two subgroups. Group 1 would like a test, but will elastically respond to any extra hassle of getting one. Group 2 absolutely needs a test and will not desist, e.g., they strongly believe they have Covid, and without a test they cannot get access to medical treatment. As tests get scarce, group 1 will gradually drop away and test takers converge to being drawn 100% from group 2. But those test-takers will obviously have lower than 100% positivity rate: if this wasn’t the case, we wouldn’t need tests at all! There are other diseases and circumstances that cause people to join group 2.
In a nutshell, positivity rate is only a proxy for undiagnosed Covid cases within some unknown operating envelope (I’m guessing 5-20%?).