Ok. Let’s use the context dependent definition of herd immunity, and look at behavior. People this time.
Spain looks to be herd immune. Still some cases, but outbreaks mostly fizzle out.
Now suppose the party scene really gets popular in Madrid this year. Too much time cooped up, and it’s time to be social. More people, more alcohol, more hookups, less distance. All of a sudden, you have spikes in a variety of diseases, including covid.
What happened? The herd immunity threshold changed. People who used to be keeping 6 feet are now adopting a different behavior. Put one way, people got closer to each other and disease spread more easily. Put another way, people changed their behavior and that altered the herd immunity threshold.