These are the hot questions at the moment. It is the hope of everyone that if you do catch Covid19, you will be immune for a considerable time. On the one hand, immunity to the common coronaviruses that cause "colds" are not long lasting, so you can catch them again as soon as four months after recovery. On the other hand, immunity to SARS lasted for up to ten years. How will Covid19 compare?

Unfortunately, it seems that antibodies peak after a few weeks, and start declining after that.
https://www.sfchronicle.com/health/article/With-coronavir...

If antibodies do fall that quickly a vaccine will most likely not offer long term protection, either, so this may turn into something that people get every year, or a couple times a year. Let's hope not.

Antibodies are not the only measure of immunity, however. Our bodies have multiple defenses, including memory B cells and memory T cells. If these persist, they may have some effectiveness in minimizing and shortening the duration of re-infections. Memory T cells are of particular interest because SARS-COV2 shares many proteins with the common coronaviruses, with which we have all probably been infected with at some point. Thus, the memory T cells may recognize SARS-COV2 immediately, and help the body fight it off quickly, which may be why so many people have mild or asymptomatic cases. It may also explain why the case totals seem to go only so high before slowing down, rather than rapidly running through the entire population. No state or country has reported more than 4% infections, which is not what you would expect if this were truly a novel virus, to which no one had any defense.

So many questions, and so few answers. Treatment options continue to improve, but we still don't know the answers to some big questions. How long does immunity last? How soon can a person be reinfected? If a person is reinfected, will it generally be more mild, or more severe?

I've heard that for many people the antibodies don't last long, so they can get reinfected. Doctors are recommending that people take precautions against Covid even if they test positive for antibodies.

Are antibodies even the main way the body fights off Covid19? Increasingly it seems that memory T-Cells are the first line of defense. When a virus invades a cell, and makes the cell begin manufacturing more virus particles, during the manufacturing, proteins being assembled are visible outside the cell. When a Memory T-Cell recognizes a protein, it either destroys the cell that is producing virus particles, or marks that cell for destruction.

T-Cells are longer lasing than antibodies, and may last for years. From testing of blood samples that were taken pre-Covid, some 40-60% of people may already have memory T-cells that recognize some of the proteins in Covid. This isn't totally surprising because SARS-COV2 shares a certain number of proteins with the coronaviruses that cause colds.

One reason that the young may fight off Covid19 more effectively is that T-cell production is related to the thymus, a tiny organ that produces a hormone that converts lymphocytes into T-cells. The thymus begins shrinking by age 30, and so after that point, our adaptive immune system becomes increasingly less effective. By 50, our bodies are making far fewer T-cells, making us much more vulnerable to Covid19, as well as to many other diseases, and cancer.

If T-cells are the longest lasting defense against Covid, this may have implications in vaccine design, as you would want to trigger the body to make T-cells. There is indication that the Oxford vaccine does trigger T-cell response, as well as antibodies:

https://www.bbc.com/future/article/20200716-the-people-wi...

On the other hand, if older people aren't able to produce large numbers of effective memory T-cells, that has implications as well, and may mean that they need a different vaccine, and/or more frequent boosters.

I know a couple people who've had it twice already.

Just saw that someone in Hong Kong who had covid-19 and recovered has caught a new strain of it.