Toddlers as young as two are part of Stanford's latest vaccine trial FacebookTwitterEmail A nurse shows off a vial of the Pfizer-BioNTech COVID-19 vaccine.Sean Rayford/Getty Images Toddlers as young as 2 years old are being immunized as part of a vaccine trial out of Stanford. The trial, part of a larger nationwide analysis on the efficacy of the Pfizer vaccines on infants, toddlers and young children, began Wednesday on a small group of 2- to 5-year olds in the Bay Area. Stanford is the only site on the West Coast to work with children; there are five sites across the country.