So each turn, B-cells are presented with the latest version of the virus, they generate various antibodies to the various parts, and are graded at the end by how well the generated antibodies bind to the virus. The problem is that you have 1 million cells which bind strongly to the fake decoy targets and 1 cell which will bind not as strong to the real effective target. So this 1 cell never gets "promoted".
What this "germline targeting" multi-shot vaccine tries to do is to introduce a series of targets that stimulate that 1 in a million cell which will attack the right part of the HIV virus, so it gets "promoted", so if the real virus appears, the body will still go for the decoy targets, but will also generate a lot of these really effective 1 in a million cells, which got "promoted" previously by the vaccine.
To be more precise, you need to "guide" a lets call it B1 cell that all of us have in our repertoire to mutate into B2 then B3 than B4, because this B4 version will be capable of creating the right antibodies for HIV, the issue being that the intermediary states, B2, B3 are not naturally promoted so you very rarely get to the B4 state without this intervention.
I would like to thank all the scientists who have dedicated all their hard work and ingenuity to eliminating the impact of cancer. Also, fuck cancer.
This is why it's hard to make a 'super vaccine' for flu, becuase it takes many rounds of shots to retrain some people's immune systems once they've latched on to the wrong recipe for antibodies.
An analogy of this is having the wrong math instruction in middle school could make your math classes harder at the university level.
It's an incredible system, and a product of probably a billion years of evolution since the first multicellular organisms had to protect themselves from infection.