Released 2 days ago, this synthesis review shows that immunity to SARS-COV2 behaves in a normal way. When a person has been quite sick and survived, they will likely have high antibody levels for that particular sickness--for a little while. These are the people from whom you might wish to get convalescent plasma, because it's probably full of antibodies that could help. But these antibodies ramp down pretty quickly once you clear the sickness. A person who had the virus months ago will not have as many antibodies in their plasma as a person who just cleared it last week. This is why there aren't studies to prove that convalescent plasma is a good treatment: it depends on the person's immune response and on the timing.
IgM is the first kind of antibody your body makes, and appears about a week into an illness. After that your body switches over to making IgG antibodies. The fact that IgM antibodies decrease after the illness is NORMAL. The IgG antibodies that are made next are the keepers. They keep being made, but at lower and lower levels if you aren't still fighting the same virus. This is also NORMAL. Your body does the same thing for ever flu it ever encounters. The fact that your body KEEPS making antibodies has to do with its MEMORY. The immune system has cells called Memory B Cells--the ones that know how to make antibodies for a specific condition--they are your body's memory. As long as your memory B cells survive, your body has a blueprint of how to make more B cells that can more antibodies to fight that remembered infection--in other words, it can respond faster.
Memory B cells can live a long time but eventually they die. The rate at which they die varies by age and lifestyle and also by what other infections you get. One really good reason to get vaccinated against Measles is that getting measles infection can kill your memory B cells, making you MORE susceptible to things that you've had before. And that stinks. I hope that you are vaccinated for measles (the MMR is available if you aren't) and that you also get vaccinated for the flu this year, because it's good to protect your lungs.
As for the COVID vaccine, we're likely to have a lot of options with varying efficacy that are available to most of us next year. I don't know for sure what will come out, but I will say that for myself I will try to get one vaccine/year, hopefully different kinds each year, to help my body build up resistance by exposure to lots of different parts of the virus. For those who are worried about adverse effects from vaccines, let me just say that they really are rare, a lot more rare than the diseases. Also you can also minimize your risk of adverse effects to vaccines by improving your ability to detoxify your body. I can help with that for sure. Spring and fall are good times for a deeper detox!