This, along with a large number of other trends related to COVID (including vaccine effectiveness), can be simply explained by immunity just not being very long

This, along with a large number of other trends related to COVID (including vaccine effectiveness), can be simply explained by immunity just not being very long lasting (4-6 months?). The Delta wave predates Omicron by around that much time. I think when all the dust settles around COVID, we're going to learn that both vaccines and "natural immunity" last 3-6 months, and that explains almost all of the noise and confusion about what "works" and what doesn't. And that also suggests that "herd immunity" was never a realistic goal, whether by vaccines or infection. Maybe some future vaccine can be developed which targets a different signal and can last longer.