People seem to always assume independent identical random variables. Corona Virus are seasonal - infections highly locally clustered.
The spreading might refuse to follow slow brownian motion of the herd. If e.g. an aircondition blasts an entire room of people with bad air, how much do herd effects really help?
Does the term "herd immunity" make sense for a seasonal virus at all?
Yes. For instance, one of the benefits of widespread flu vaccination is that it slows the rate that the virus spreads at. Herd immunity is just the threshold where enough people have immunity that the spread diminishes. That we don't necessarily reach the threshold doesn't make the lesser effect useless.
So herd immunity is not about immunity. Quite a misnomer then! Always thought herd immunity protected those still vulnerable, but according to you it merely means the vulnerable may merely be having a few more healthy hours falling sick non the less.
Low level zero friction school physics analogy: think of a ball rolling onto an inclined plane. If the plane is angled upwards the ball will slow down and eventually stop rolling further upwards, if the plane is angled downwards it will just continue accelerating forever. But in the upwards case no place on the plane is truly safe from getting reached by the ball, because speed and angle is not specified. Herd immunity means that the slope is upwards, but the velocity would still be an open-ended random distribution.
A more accurate model for virus spread would replace momentum with dice rolls and slope with success thresholds, but that's just more difficult to imagine (unless your biography contains a phase of obsessing over p&p rpg rules).
An individual has immunity when the immune response is triggered. By definition. It was very confusing to me at first before I caught on to the meaning of the term.
It makes sense when you think about it. Can you "get" the virus a second time? Of course of can. That doesn't contradict the fact that you have some level of "immunity".
After all, the immune response is a big part of what most of us feel when getting sick.