Replication-competent smallpox vaccine consists of a live, infectious vaccinia virus that can be transmitted from the vaccine recipient to unvaccinated persons who have close contact with the inoculation site, or with exudate from the site. The risk of side effects in household contacts is the same as those for the vaccine recipient. Therefore, the vaccination site requires special care to prevent the virus from spreading.
from here
https://www.cdc.gov/smallpox/clinicians/vaccines.html
I can remember mom taking care of it, the blisters, and being told not to scratch. Not fun but better than having smallpox.