This is
part of why we're never getting an effective vaccine for this thing.
Confirmed case of true reinfection, beyond any doubt because it was two different strains. Antibodies for the first infection clearly didn't work to combat the second infection, because his two bouts of illness were within a month of each other, when post-infection antibodies should be highest. (Which is to say, a hypothetical vaccine that worked for the first strain wouldn't have worked for the second, and if you can get as many new strains as they're seeing in just 9 months of spread in the U.S., you'll never stay ahead of it.)
The first infection was mild, the second put him in the hospital. He was 25 years old, in good health, no autoimmune or pre-existing conditions.