I don't think they have started yet, but such trials are planned in the UK.

There's no easy blanket "unethical" ruling from on high. Obviously this isn't a risk-free thing to volunteer for, but it isn't risk free for nurses to go to work either, and thankfully, they still do.

Also, you'd possibly be giving an unrealistic dose in the lab. If you give someone 1000x the viral load they'd ever get in the real world, the vaccine might not work. Or might work but send the immune system into a crazy overdrive cytokine storm.