And if you get it wrong, you now have a second pandemic to fight.
If it's incompatible with some people, bad luck for them, can't stop using it on them after discovery.
TL;DR: not easy and basically impossible to do ethically
Your idea reminds me of using a computer worm to patch backdoors. I'm sure I've read of this before.
This mindset is very dangerous in my opinion.
People need to have consent. There are people who cannot recieve vaccines due to medical reasons. Would you just release something without people's consent that could kill and injure a portion of them?