Here’s what Johns Hopkins Patient Guide for the immunocompromised says: “Avoid contact with children who are recently vaccinated,” and “Tell friends and family who are sick, or have recently had a live vaccine (such as chicken pox, measles, rubella, intranasal influenza, polio or smallpox) not to visit.” St. Jude's Hospital’s website goes even further, warning parents not to allow visitors to children undergoing cancer treatment if they have received oral smallpox or polio vaccines within the past four weeks, if they have received the nasal flu vaccine within the past week, or if they show any sign of a rash after receiving the chickenpox vaccine or MMR (measles, mumps, rubella) vaccine.