Most likely. You need to develop antibodies to defeat a viral infection and, once you do, they don't just disappear overnight.

Well, as far as I know, the antibodies do disappear at some point, but now your immune system knows the virus and can quickly produce new antibodies again, if it finds the virus again. But I think over time the immune system also forgets.

But to the patients that got sick again. "Bad test" would be the best outcome yes, but I know, that with herpes virus for example, you can never really get rid of them. Your immune system defeats them, but some virus cells remain in stasis somewhere sleeping - and resurface, if your stress level is high enough. So it would not surprise me, if this coronavirus might have similar traits.

https://en.wikipedia.org/wiki/Herpes_simplex

Something I read recently is that the immune system is complex containing both an innate immune system an adaptive immune system. It's possible that in mild cases of Covid-19, it may be successfully handled by the innate immune system, which doesn't confer additional immunity. A mild case my occur if the initial infection is by a relatively small number of virus particles.

Also possible yes. The killer cells (do not know the proper english name) are general purpose defense cells against anything that the body does not know. Only if there are too many unkown cells of similar type, they get categorized by other cells in the lymphs of the immune system and then other cells finally produce antibodies. But that process takes time.

>The big "eating" cells (do not know the proper english name)

macrophage

Ok, I edited eating cells to killer cells, before you replied. Because with a quick look on english and german wikipedia, I found a bit contradicting information. Anyway, my first post was more right as the macrophage cells are indeed a first line of the defense and killer cells come later (ca. 3 days).

So .. I leave this debate to more knowing persons.

>macrophage

That's arguably Greek ;-)

Not really. THe innate immune system "talks" to the adaptive system. Innate immune cells are often incredibly proficient at displaying antigens to the adaptive immune system and initiating the adaptive immune response.

Innate immunity is not going to learn about Covid-19. That adapts on an evolutionary time scale.

We share Toll Like Receptors (TLRs) homology with drosophila. That's how ancient innate immunity is.

Herpes virus are entirely different from COVID and remain "sleeping" via a mechanism that simply cannot happen with a coronavirus.

Good to know, but is this secured knowledge? I thought there is too much not understood about COVID?

We don't know a lot about the pathology of COVID, like why is it so much worse than the coronaviruses that already circulate around the world and cause mild flu-like symptoms. We know a ton about coronaviruses in general, especially basic issues about how they replicate. Herpes viruses use a number of different genes to regulate latent infections. These genes are simply not found in COVID or any coronaviruses. This is not something we could be misunderstanding/missing. It's possible, but would be HIGHLY unlikely, that COVID has some, as yet undiscovered and never before seen in any other coronavirus studied to date, method of achieving latent infection and reactivation.

How come you can get the flu twice?

This makes no sense. It's not the antibodies, it's the virus itself - it can mutate.