Low viral load exposure will likely increase the chances of a milder progression. But that's a very risky gamble. Will still be way too lethal. It is still unknown if this effect is insignificantly small.

Instead we could grow lots of viral particles. Irradiate them to certain death. Then use it to repeatedly challenge the immune system over a long period. Very expensive and nowhere near as effective or reliable as a proper vaccine, but would induce some level of resistance in a fraction of the population.