Variants have no impact on immunization durability. A specific vaccine, for a specific variant, only has a finite immune effectiveness within the body. For example, a tetanus shot provides 96 effectiveness for 13-14 years, but tht dips to 72% at 25 years as such they recomend getting one every 10 years. The effectiveness of the mumps vaccine is 10 years before it starts to diminsh.
Flue vaccines change yearly because the virus adapts and a new vaccine are needed for new variants, however you are still protected from the original variant that you were immunized against for many many years.
Efficacy is how effective a vaccine is when measured at a specific point in time. Durability is a measure of efficacy over a period of time. So while the covid vaccine may have a 95% efficacy at 30 days, it may only be 70% at 1 year, and 40% at year two. We don't yet. I suspect we have a good idea that the short term durability isn't what we want to see, but we're purposefully not sharing that data because, well, we need people to get the shot, even if it's only effective for 3 months. Some of the studies that are slowly coming out are suggesting the durability is actually much less than that. (The image above) Meaning a yearly booster, or even more, will almost certainly be required. If the virus adapts to a variant that's unphased by the current vaccine, then they will need to develop a new one and offer it also, much like the flu vaccine. This will also vary by age group. Kids have a stronger immune system and immune memory than older adults. It's not uncommon for vaccines to be far less durable in older adults than kids.