Israel, with it's 58% vaccinated, never achieved herd immunity (especially not for the delta variant) so that doesn't prove anything.
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And degradation isn't as simple as antibody titers. I've noticed that the media never accounts for memory b and t cells. b and t cells have kept the efficacy for severe covid and deaths the same.
The fundamentals for herd immunity remain the same whether you accept it or not. The polio/chicken pox vaccines had to abide by the same mathematical rules as the coronavirus vaccines. It's not just about vaxxed breakthrough cases, but instead how many breakthrough cases transmit to others in conjunction with how transmissible their respective viruses are.
80% of vaccinated COVID carriers didn’t infect anyone in public spaces — report
And for the record, chickenpox is way more transmissible than the delta variant and the vaccine for it has a lower efficacy than Moderna, Pfizer, and J&J. If you can get herd immunity in that scenario then you can reach herd immunity with this current pandemic.
Science changes. A new variable was thrown in the mix in the form of the delta variant which had a higher R0 which transitively raised the threshold for herd immunity.