Probably because once the cultured virus is taken away from the host and put into the vaccine, it can no longer reproduce inside the recipient of the vaccine (u

Probably because once the cultured virus is taken away from the host and put into the vaccine, it can no longer reproduce inside the recipient of the vaccine (unless the recipient happens to express the VP30 gene, which had to be engineering into the monkey kidney cells anyway). So virus in the vaccine can cause only minimal damage because it can't reproduce.