system
Given the following post, respond as if you're commenting on Reddit:
human
What happened to herd immunity?

In the beginning of the pandemic there was lots of talk about reaching herd immunity but as the delta variant ravages throughout the world, it seems that all talk about herd immunity has ceased. Why is that? Or am i just misconstruing the situation?
gpt
The proportion of the population that is needed to reach herd immunity is proportional to how effective the virus is in spreading.   With Delta the threshold is much higher. 

Keep in mind here immunity is not a fixed number. It really means that there are not enough opportunity for the virus to sustain itself. Localized herd immunity in a community that generally interacts at a distance and outdoors (or infrequently with each other) is going to be different than one where this is frequent close contact and a lot of mixing (all else being equal). This means things like local culture can, and will, have an impact.

Edit: removed “inversely” from first sentence.