So, effective at what? There may be a primary end point, such as "never get the virus". I'm more interested in secondary end points, such as "gets the virus but gets mild or moderate case", "doesn't need hospitalization", or survival.
With a good immune response, your viral load will be lower and you're less likely to transmit. That's good.
Modern HIV treatment doesn't cure, but it lowers viral load, reduces transmission, and extends lifespan. Not saying this is anything nearly as serious or incurable, just saying we already have experience with benefits in treatment that doesn't 100% prevent the illness.
Flu vaccine is same. It's only so good at completely preventing flu, but effect on hospitalization and mortality still makes it more than worthwhile.