You are referring to Shedding.
This is actually a real concept that has been warped by the anti vaxxers.
In short shedding can happen in a live-attenuated vaccine. (A vaccine contains an actual weakened virus.)
In such a case the virus multiplies in the body and can spread to other people in its weakened state.
True, afaik mRNA are not live-attenuated vaccines. Which makes the whole thing so insane.
I can understand a person saying it’s a new vaccine with new technology being a little worried, I may not agree but there is some logic to that.
What gets me, is when there is no logic just hysteria.