The mark of the beast cannot be a vaccine. Since those with the mark are damned, and irretrivably so, accepting the mark of the beast must be a mortal sin and a deliberate choice or act against God.
In the case of a vaccine there would be stupid, ignorant, mentally handicapped people who would be ''marked by the beast' thinking they were just being vaccinated against the Kung Flu. God would therefore have to damn them to Hell for being ignorant.
Revelation is clear that no-one with the mark of the beast is saved. So a vaccine cannot be it.