The mark is not the vaccine itself.
What COULD be the mark is means by which one proves he RECEIVED the vaccine.
Someone would have to be quite foolish at this point not to see we are quickly moving toward mandatory proof, in the form of vaccine passports, in order to buy and sell EVERYWHERE.
At first, the paper vaccination cards will do. But that will quickly move toward a required phone app. They will cry that the paper card is too easy to forge.
Then they’ll cry that the app is too easy to hack.
The next step will be some form of bio metric mark that a person receives on their hand or forehead that can be read by a scanner or even phone proving they got the vaccine.
There are several candidates ready to be rolled out right now.
Like this:
https://www.mintpressnews.com/africa-trust-stamp-covid-19-vaccine-record-payment-system/269346/
Or maybe Mr Gates quantum dot tattoos that use luciferase.