In this last part of our medicines and vaccines series, we’re going to talk about how vaccines work and how we know they do. I know that vaccines are a contentious topic and I want to start by acknowledging that sometimes delaying or forgoing a vaccine is the evidence-based decision. If someone is pregnant, they shouldn’t have the varicella (chickenpox) vaccine. A child with leukemia probably shouldn’t get the measles, mumps, and rubella (MMR) vaccine. Those conditions which make vaccines temporarily or permanently unsafe for an individual are, thankfully, very, very rare. For most humans, following the CDC vaccine schedule is safe and will provide both them and their contacts with protection against those diseases.