Example of Herd Immunity.... Various flu seasons have higher impact to younger folks vice older folks who had already developed natural immunity because of getting similar flu years ago — when they were young.
From a random selection of web pages via search
https://www.ecdc.europa.eu/en/seasonal-influenza/prevention-and-control/vaccines/immunity
I know what herd immunity is.
I asked you about human coronaviruses, and why you believe herd immunity is possible with this group of viruses.