https://web.archive.org/web/20201101161006/https:/www.who.in...
https://www.who.int/news-room/q-a-detail/coronavirus-disease...
Herd immunity is a moving target because people like Fauci change their assessment of it depending on opinion polls. That's by his own words, not anyone else's. The guy admitted it to the New York Times. The "science" is not evolving and people who keep claiming it is, are just hurting the credibility of actual science even more than it's already been hurt. After all, a "science" that is constantly asserting things with 100% confidence and then next week asserts something totally different, also with 100% confidence (which they do), is actually pretty worthless to society.
Also, the threshold for herd immunity is based partially on how well the virus spreads, and Delta changed that, which means that the threshold for herd immunity probably changed. When facts change, the messaging needs to change to stay accurate.