Well, a virus can be contagious without symptoms. COVID in particular starts spreading before symptoms suggesting you could have respiratory virus without people even being aware they are infected. And we know most people are not even aware they were sick or they had only mild symptoms.

The issue with COVID is that some people are hit hard and a lot of people develop hard to detect complications.

If you could modify the virus to avoid complications and avoid hitting some people very hard, you could have pretty effective way to innoculate population.

The thing you are after is a virus that has same basic spike protein for your immune system to recognize and only enough mechanism to multiply itself.