Great news. A number of studies reporting in the last few weeks indicate that the T-cells of 4 available vaccines all provide strong protection against the COVID virus, independent of variant and it is thought that all four could handle any future variant. Also the B-cells produce more robust anti-bodies over time that become more likely to handle a previously unseen variant. Indications are that the protection provided by the vaccines could last much longer than we had dare hope earlier. The same seems to be the case for protection provided by infection, that you are protected for a longer time period than earlier thought to be the case.
The reporting is a little confusing, complicated, so read it yourself. It will probably take reading the papers to really understand the ins and outs.
What occurs to me immediately is why do some people get recurring cases? This is not exactly uncommon. I suspect it has to do with their specific genetic make up.