If young people get infected, clear their infection without detectable antibody responses, then does that mean they are not immune? I don't know. Nor does anyone I suspect, quite yet.
If they get reinfected at a later date , 6 months or more, will they have the same benign illness they had initially, or will it return significantly more severe the second time like can happen with dengue? Again, I don't know, nor does anyone else yet.
There is still much to learn about the Covid 19 virus. One factor that is not understood yet is how many safe harbors it will find - There is some evidence that it can infect tigers, lions, and house cats. If this is true, that might be a real concern - we're not really likely to interact closely with lions or tiger ( the Netflixs Tiger King notwithstanding ) but domestic cats live in lots of folks houses and some roam neighborhoods at night without surveillance. Can humans catch Covid 19 from domestic cats - who really knows yet? But maybe???....
Typically after a viral infection one gets a spike in immune globulins within 2-4 weeks which gradually falls unless one is re-exposed to the virus. So this pattern of minmal Ab titer seems unusual to me but I am not a virologist or immunologist. We need more authoritive answers I think. They will be forthcoming but it may take a while. The folks at Stanford may have some answers they haven't published yet.
One interesting point in the article I linked is the possibility of ivermectin inhibiting viral replication of Corana virus.
Thinking along these lines, reminded me of a virus exposure of mine back in 2001. After September 11 and the destruction of the towers in New York City, the excitement didn't stop then - someone began to mail around weapons grade anthrax spores, and there apparently was some serious concern that small pox might be added to the viral exposures - Small pox was offically declared extinct in human populations by WHO on May 7 , 1980.
Despite the fact that no small pox was in human populations, there were two small samples retained - one by the USA and one by the USSR. Physicians were quietly polled, nationally, to see who would be willing to examine potential smallpox victims, and specifically they were looking for ophthalogists who could biomicroscopically exam human corneas for signs of small pox. I volunteered as I KNEW I was successfully vaccinated for small pox as a young child ~ 55 years earlier. One of the requirements for being accepted as a volunteer, was that I had to be willing to be re-vaccinated for smallpox again.....
I said sure, I had great faith in my childhood vaccination.
So I reported to my local county health nurse who pulled out a tiny glass vial from the 1950s containing viable vaccina virus and proceeded with scarification - multiple tiny punctures in my skin to allow the virus access to my tissues. She put a bandaid on it and off I went home.
I didn't think too much about it until about 36 hours later when the viremia poured through me and my temperature rose to 101 or higher and I felt like I was getting a very very bad case of the flu. This viremia lasted less than one day and my immunity popped up and I fully recovered. But that viremic phase was more severe than any flu I have had in any of the following 18 years since.
I still remember it, and the brief, but temporary, seconds thoughts I had about the wisdom of volunteering....
Reading about small pox is still sobering, and ought to be required of all who deny the great value of vaccination. Almost nothing medicine has done in the last 200 years, has saved as many lives as vaccination, starting with Jenner, and vaccination with cow pox virus.