I am not a doctor. But I kind of doubt it—if your immune system would “forget” about coronavirus, it would also forget about every other disease it’s ever encountered. This means that people would get very sick after fasting and that other vaccines (like tetanus or polio) would not work, but neither of these things is true. So I don’t think that immunity “memory” works in that way.
Fasting will have little to no impact on effectiveness of covid vaccine. Immunity is vastly complex. Fasting can reduce levels of inflammation activity that are not necessary, but your body will still produce good inflammation when prompted by a foreign substance. This is good.
Analogy: a peace treaty doesn’t mean your country no longer has a military. It means you dont fight needless wars.
No, because your immune system has memory cells that don’t get “wiped out” because even the cells that have memory if they get “wiped out” the antibodies would deattach and the “new” cells would in a sense get the antibodies and you’ll stay vaccinated