Except that I don’t believe ANY of the numbers put out by “official” sources - they had an agenda - By the time the vaccines were available - Alpha was already no longer prevalent (and was indeed gone in most areas of the US - )
Delta was born in Uttar Pradish, India, Dec 2020, spreading to England before jumping to the US about May 2021 (first 2 cases TX) So there were several months after vaccine rollout where Alpha was by its lonesome - long enough to get most of the seniors vaccinated. The first version of Delta reduced vaccine efficacy by anywhere from 10-20%.
Then around the end of 2021, Omnicron surfaced, circulating with but outcompeting Delta. By March 2022, omnicron BA.2 was circulating in China. It’s suspected BA.2 is being outcompeted by BF.7 (a derivative of BF5). BF=7 is bad news, defeating most immunity, and with an RO of 10 to 18, compared to original Omnicron with an RO of about 5.
https://www.livescience.com/covid-what-we-know-about-new-omicron-variant-bf-7
as to boosters, it could be that younger people are more prone to get ‘immune exhaustion’ than older folks who have weak immune systems and so boosters are more effective for them. from April, 2022: “..there may not be meaningful benefit of a second booster of the same formula for young, healthy populations, “ but for those over age 60, “.. the second booster equated to a 78% reduction in death compared to the first booster alone..”
78% is a substatial number of lives saved.
https://theconversation.com/do-you-need-a-second-booster-shot-an-epidemiologist-scoured-the-latest-research-and-has-some-answers-180488