Like previously stated, HIV hides inside of cells for an indeterminate amount of time. As you mentioned immune memory can take a little bit of time to kick in. Once it kicks in however, it is extraordinarily effective at hunting down viruses and actively (non-dormant) infected cells. With a small enough viral load however, the immune system can fend off HIV well enough.
During an initial infection either the immune system will fight off the virus if the load is small enough and it gets lucky or the virus takes hold of some cells and starts reproducing. In this latter case, the immune memory allows the immune system to effectively fight off the virus before it can properly take hold. Of course some of the virus may lay dormant in a few cells but since it never gets a chance to take hold of the body the quantity of dormant infected cells is relatively low.
Now due to the low quantity of dormant infected cells, it is extremely unlikely that there will be enough "activated" cells at any given time that the immune system is not able to handle the threat before it escalates. Over time you can expect the dormant HIV to slowly be exterminated or at very least prevented from growing in count.
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Now this isn't anything terribly new. The real breakthrough with this vaccine over other attempts in the past is that it results in the production of a specific type of antibody that can act on all known HIV strains with essentially the same efficacy (where as prior vaccines couldn't result in the production of antibodies that worked reliably on even small portions of the thousands of different HIV strains).
>Right but for a virus that integrates into the genome once it takes hold it's game over.
The same could be said about any other successful vaccine, like Polio and Measles vaccines. Not that those diseases behave like HIV, but the way we handle vaccination against them.
>And even vaccine mediated immune responses are via memory cells and take time to ramp up to generate the antibodies to attack the virus.
That's why there are vaccination campaigns, to immunize target groups before they contract the aforementioned sickness.
>Is the window of opportunity while ramping up the immune system for a vaccine I mean unresponse big enough for HIV to take root in a body.
A vaccine is always a good opportunity to prevent adverse effects, even "if takes time" to ramp up your immune system.
Afaik vaccines are not useful after the fact. I.e. this HIV vaccine won't be given to people already infected.