The doses do not all set your immune system to the same state, despite being the same substance. The subsequent doses cause further evolution within the body to

The doses do not all set your immune system to the same state, despite being the same substance. The subsequent doses cause further evolution within the body to broaden the spectrum of its response. This is why there were two doses in the first place, on a schedule, instead of one dose. The sequence is more like a progressive immune system training program than a one-off health powerup like you'd get in a video game. Omicron is close enough that the figures show a significant effect after the third dose compared with the second. It was not something which could be assumed, because the response curve depends on details of the virus, so the recent data is good news. However the scientists had enough understanding of the details, and Omicron spreads so fast with unknown consequences at this time, that it was worth rolling out the booster soon after Omicron was detected. It is a popular misconception that each vaccine sets a simple "immunity %" stat, and that the booster simply resets that stat after it wanes. It's an easy idea to explain, but the immune processes are more subtle than that. Perhaps we need some of that subtlety to be better known now, so that people understand why the booster sequence is useful. https://www.bbc.co.uk/news/health-59639973.amp https://www.bbc.co.uk/news/health-59639973.amp