COVID-19 itself, but also vaccines "change" the immune system permanently. Normally, the disease itself triggers the same immune response as the vaccines do. This means that there wouldn't be any further benefit as a long hauler to take the vaccines. But theoretically, it's possible that a subgroup only has an adaptive response against the disease and not much of an innate response and that the vaccines could further improve the innate response (or vice versa). But this is a controversial topic, which is discussed in the context of the original antigenic sin. I'm still waiting for more data on the long-term effects of the vaccines in order to pick which one might provide the greatest benefits for CFS/ME patients with frequent viral reactivations.