I sold a vaccine I knew didn't work in order to make money
That is horrible. Why not just give them a real vaccine?
It's too late to stop now. It's already sent in the mail to the customer
Selling items that are fake is illegal and deceptive. Notify the recipient of what you have done and make amends by sending them a real vaccine.
I'm not going to say anything to anyone.
If they take it and something bad happens to them then you will be the one to blame. The authorities will find where it came from and lock you up if you don't stop this now.