There is a vaccine against polio that can infect and immunize others. It uses a live polio virus genetically modified to not cause severe disease.

"The OPV is less expensive and easier to administer, and can spread immunity beyond the person vaccinated, creating contact immunity. It has been the predominant vaccine used. However, under conditions of long-term vaccine virus circulation in under-vaccinated populations, mutations can reactivate the virus..."

from https://en.wikipedia.org/wiki/Polio_eradication

Interesting, like a fully engineered virus with the same spike proteins, but none of the "bad stuff"? Seems tricky, I think in order to be contagious, it needs to replicate, which is part of what makes viruses so harmful, right? Even if you can make it harmless, replicating means mutations will happen, so that's an interesting new kind of risk to take.

I had the same idea and was convinced by a biologist, that his is a very bad idea. To be able to spread, this vaccine-virus needs to infect the people vaccinated. Any infection is dangerous, as it means the virus is multiplying in your body until the immune system hopefully takes hold of it. What happens if it doesn't? This happens with traditional vaccines all the time, but the consequence is only that you are not acquiring immunity, no other harm done.

The other point is, you have made a completely new human-transmissable virus. How do you reliably prevent it from mutating into something more dangerous?

Interesting. That's not a vaccine anymore that's a strain of SARS-CoV-2 that's never produces symptoms, but is still similar enough to the original to prime the immune system. Presumably it would out compete the original in a Darwinian sense, because people who caught it would never get sick and stay home.

Risky, though, because if it turns out to be less benign than we thought - or mutates in such a way as to become lethal - we've just started a second pandemic.